Add ConstantArgument support to fx_import

[alaa-ali]

This PR will fix the following issue: fx_importer NotImplementedError: MultiheadAttention layer with NeedWeight = false

The following error was raised before this fix: Python Error: NotImplementedError: OutputKind.USER_OUTPUT for <class 'torch.export.graph_signature.ConstantArgument'>: ConstantArgument(name='', value=None)

This occurs for an exported MultiheadAttention layer with "NeedWeight = false" which means weights are not going to be returned by the layer. So, the second output attn_output_weights will be None in this case.

[penguin-wwy]

@stellaraccident plz review this

[stellaraccident]

Looks like test are failing. Let's talk about this if it is due to a too old version of pt in the CI.