adjust overriding of model's forward function

[prathikr]

During ONNX Runtime optimization, the forward function's signature is inspected to perform downstream tasks. The current way of overriding the model's forward function results in the following error:

AttributeError: 'function' object has no attribute '__func__'

This PR improves the function signature override by using types.MethodType which doesn't impact the function signature for downstream tasks.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

LGTM but would love to have @muellerzr have a look as well.

Edit: looks like this breaks in mixed precision settings.

[muellerzr]

The solution/idea presented here makes sense, but let's make sure tests pass before we can merge please :) (And then I'll give the ✅ 😉 )

[muellerzr]

Thanks for this! Great work! Let's do a quick make style; make quality to ensure that the quality tests pass, then we're good to merge :)