Fix activations being all the same module
# What does this PR do?

Since #15616, all activations of the same type are the same instance of the same object. This seems harmless at first glance, but cause issues when hooks are automatically added to models, for instance in the big model inference. In this case all objects share the same hook (since they are all the same) leading to subtle bugs (kudos to @ArthurZucker and @younesbelkada for finding the root cause!)

This PR changes the `ACT2FN` dictionary to return new instances of the activation classes each time you provide a key.
_The documentation is not available anymore as the PR was closed or merged._