Refactor classes to inherit from nn.Module instead of nn.Sequential

[amyeroberts]

What does this PR do?

Refactors classes that inherit from nn.Sequential to inherit from nn.Module instead. This is to make the code easier to debug and inspect.

Changes:

• Explicit forward method implemented for classes
• Iterating over layers and them registering to the module using add_module(str_ind, layer) in __init__. This provides backwards compatibility as the submodules will be named according to their position in the call stack, like in nn.Sequential, which is needed to load in the same checkpoints.

Note: This does not include other possible nn.Sequential refactoring within modules e.g. lxmert, or inheriting from ModuleList e.g. in Beit. These are more involved changes and should be address in separate PRs.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed.

[HuggingFaceDocBuilderDev]

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

[sgugger]

Thanks a lot for working on this! I don't think we will remove the use of nn.Sequential inside a module so this case should be left as is.

[LysandreJik]

Great, thank you @amyeroberts!