[SYCL][Graph] Add spec wording for dynamic events

- Adds dynamic events which are updatable
- Removes limitations on events form outside a graph
- depends_on prop can now take events
- Added get_event to get event for node in graph

sycl/doc/extensions/experimental/sycl_ext_oneapi_graph.asciidoc

@@ -281,11 +281,14 @@ represent data dependencies between two command groups captured as nodes.
 Secondly, by using the `handler::depends_on()` mechanism inside a command group
 captured as a node. However, for an event passed to `handler::depends_on()` to
 create an edge, it must be an event returned from a queue
-submission captured by the same graph. Otherwise, a synchronous error will be
-thrown with error code `invalid`. `handler::depends_on()` can be
-used to express edges when a user is working with USM memory rather than SYCL
-buffers. Thirdly, for a graph recorded with an in-order queue, an edge is added
-automatically between two sequential command groups submitted to the in-order queue.
+submission captured by the same graph. Passing events from other sources (other
+graph submissions, regular SYCL submissions) will not create edges in the graph,
+but will create runtime dependencies for a graph node on those other events.
+`handler::depends_on()` can be used to express edges when a user is working with
+USM memory rather than SYCL buffers. Thirdly, for a graph recorded with an
+in-order queue, an edge is added automatically between two sequential command
+groups submitted to the in-order queue. 
+
 |===
 
 ==== Sub-Graph
@@ -565,16 +568,24 @@ class depends_on {
   public:
     template<typename... NodeTN>
     depends_on(NodeTN... nodes);
+
+    depends_on(const event& ev);
+
+    depends_on(const std::vector<event>& events);
 };
 }
 ----
 
 The API for explicitly adding nodes to a `command_graph` includes a
 `property_list` parameter. This extension defines the `depends_on` property to
-be passed here. `depends_on` defines any `node` objects for the created node to
-be dependent on, and therefore form an edge with. These nodes are in addition to
-the dependent nodes identified from the command-group requisites of the created
-node.
+be passed here. `depends_on` may be used in two ways:
+
+* Passing nodes from the same command_graph which will create dependencies and
+graph edges between those nodes and the node being added. 
+
+* Passing SYCL events will create runtime dependencies for execution of the
+graph node but will only create edges if those events are associated with other
+nodes in the same graph.
 
 ==== Depends-On-All-Leaves Property
 [source,c++]
@@ -646,6 +657,8 @@ public:
 
     void update(node& node);
     void update(const std::vector<node>& nodes);
+
+    event get_event(const node& n);
 };
 
 }  // namespace sycl::ext::oneapi::experimental
@@ -761,6 +774,15 @@ dynamic parameter for the buffer can be registered with all the nodes which
 use the buffer as a parameter. Then a single `dynamic_parameter::update()` call
 will maintain the graphs data dependencies.
 
+===== Node Event Dependency Update
+
+Event dependencies for nodes can be updated using <<dynamic-events, Dynamic
+Events>> in a similar usage to Dynamic Parameters.
+
+Event updates are performed using a `dynamic_event` instance and calling
+`dynamic_event::update()` to update all the associated event dependencies of
+nodes which the `dynamic_event` is associated with.
+
 ==== Graph Properties [[graph-properties]]
 
 ===== No-Cycle-Check Property
@@ -1094,6 +1116,15 @@ std::vector<node> get_root_nodes() const;
 ----
 |Returns a list of all nodes in the graph which have no dependencies.
 
+|
+[source,c++]
+----
+event get_event(const node& n);
+----
+|Returns a SYCL event which represents the completion of node `n` which is valid
+only for the next execution of the graph. This event can be used as a dependency
+in the same way as normal SYCL events.
+
 |===
 
 Table {counter: tableNumber}. Member functions of the `command_graph` class for
@@ -1571,6 +1602,89 @@ a normal SYCL command-group submission.
 associated with the graph node resulting from this command-group submission is
 different from the one with which the dynamic_parameter was created.
 
+|===
+
+=== Dynamic Events [[dynamic-events]]
+
+[source,c++]
+----
+namespace ext::oneapi::experimental{
+  class dynamic_event{
+    dynamic_event();
+    dynamic_event(const event& ev);
+
+    void update(const event& ev);
+  };
+}
+----
+
+Dynamic events represent SYCL events from outside of a given `command_graph`
+(either obtained from normal SYCL operations or from another `command_graph`)
+that nodes in that graph may depend on.  Dynamic events also allow for these
+dependent events to be updated between graph executions.
+
+Dynamic events can be used to add dependencies to a graph node in the same way
+that regular SYCL events can, by passing them as parameters to
+`handler::depends_on()` inside the CGF which represents the node.
+
+[source,c++]
+----
+// Obtain an event from a normal queue submission
+event OutsideEvent = queue.submit(...);
+
+// Create a dynamic event to wrap that event
+ext::oneapi::experimental::dynamic_event DynEvent {OutsideEvent};
+
+// Add a graph node which depends on that dynamic event
+Graph.add([&](handler& CGH){
+  CGH.depends_on(DynEvent);
+  CGH.parallel_for(...);
+});
+----
+Dynamic events created with an event from a `command_graph` cannot then be
+associated with other nodes in that same graph as this would change the shape of
+the graph. Attempting to call `handler::depends_on()` with such a
+`dynamic_event` in that situation will result in an error.
+
+Dynamic events can be created with no event but must be updated with a valid
+event before any executable graph which depends on that event is executed.
+Failing to do so will result in an error.
+
+Table {counter: tableNumber}. Member functions of the `dynamic_event` class.
+[cols="2a,a"]
+|===
+|Member function|Description
+
+|
+[source,c++]
+----
+dynamic_event();
+----
+
+| Constructs a default `dynamic_event` which is not associated with any SYCL event.
+
+|
+[source,c++]
+----
+dynamic_event(const event& ev);
+----
+
+| Constructs a `dynamic_event` which is associated with the SYCL event `ev`.
+
+
+|
+[source,c++]
+----
+void update(const event& ev);
+----
+
+| Updates the SYCL event associated with `dynamic_event`. This update will be
+reflected immediately in the associated modifiable graph nodes. An executable
+graph can then be updated to reflect these new event dependencies using
+<<executable-graph-update, Executable Graph Update>>.
+
+
+
 |===
 
 === Thread Safety
@@ -1646,26 +1760,6 @@ of failure. The following list describes the behavior that changes during
 recording mode. Features not listed below behave the same in recording mode as
 they do in non-recording mode.
 
-==== Event Limitations
-
-For queue submissions that are being recorded to a modifiable `command_graph`,
-the only events that can be used as parameters to `handler::depends_on()`, or
-as dependent events for queue shortcuts like `queue::parallel_for()`, are events
-that have been returned from queue submissions recorded to the same modifiable
-`command_graph`.
-
-Other limitations on the events returned from a submission to a queue in the
-recording state are:
-
-- Calling `event::get_info<info::event::command_execution_status>()` or
-`event::get_profiling_info()` will throw synchronously with error code `invalid`.
-
-- A host-side wait on the event will throw synchronously with error
-code `invalid`.
-
-- Using the event outside of the recording scope will throw synchronously with error code
-`invalid`.
-
 ==== Queue Limitations
 
 A host-side wait on a queue in the recording state is an error and will