[Export] Remove op filed from ex.Node schema (#93208)
Node can only be 'call_function' ops
'placeholder' and 'output' are serialized as inputs and outputs of the Graph
'get_attr' is not needed anymore, as it's an implicit lookup from GraphModule's parameters/buffers
'call_method' and 'call_module' is not supported, as it's not used in the canonical FX Graph
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93208
Approved by: https://github.com/suo, https://github.com/Neilblaze
