[torchcomms] Simplify ParallelDims to use base class inheritance and mesh views

[mori360]

torchcomms CI fails(we fail to close comms because there's no comm in self.parallel_dims.comms) due to the change at #1660 that we remove

    def build_mesh(self) -> DeviceMesh:
        if self.ep > 1:
            return self._build_mesh_with_ep()
        else:
            return self._build_mesh_without_ep()

So that self._build_mesh_with_ep and self._build_mesh_without_ep in TorchCommsParallelDims is not called.
This PR combine them together and just call them under TorchCommsParallelDims.build_mesh, so that TorchCommsParallelDims is built successfully, so that we can finally close the comms.