triton_ce executor: don't error if triton is not installed (PR2466)

[kshitij12345]

Similar to nvfuser_ex, we default triton_ex to be None and we register a triton_ex only if triton is available.

[carmocca]

This is great. You should be able to remove

lightning-thunder/thunder/extend/__init__.py

Line 314 in f37ac34

from thunder.executors import triton_crossentropy

and update

lightning-thunder/thunder/tests/test_extend.py

Line 130 in f37ac34

actual.discard("triton") # remove when triton can always be imported

[t-vi]

As Carlos says, this is great. Thank you @kshitij12345 , @carmocca .