Add type annotations for segformer pytorch

[p-mishra1]

This PR adds type annotation for segformer pytorch as mentioned in #16059.
@Rocketknight1

[HuggingFaceDocBuilderDev]

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

[Rocketknight1]

This looks great, thank you!

[FrancescoSaverioZuppichini]

I believe there are some minor mistakes in the types. Optional[...] is a shorthand notation for Union[..., None], see python doc. Thus it must be used when parameter : ... = None.

This PR makes Optional the following parameters in this forward method

output_attentions: Optional[bool] = False,
output_hidden_states: Optional[bool] = False,
return_dict: Optional[bool] = True,

While to be correct with the Optional definition given in python.

output_attentions: bool = False,
output_hidden_states: bool = False,
return_dict: bool = True,

Since the code in the forward method doesn't check output_attentions is not None, to my understanding they should not be Optional.

cc @Rocketknight1