Update return introduction of `forward` method
# What does this PR do?
Make it clear that the `forward` method now returns a dict instead of tuple.

PR #8530 switch the default value of `return_dict` in configurations to `True`. This caused some older code that relied on `return_dict` being set to `False` to break since assigned a dictionary to tuples can have unexpected outcomes such as receiving strings instead.

The phrasing of the introduction under the return section of the `forward` method currently state that a dictionary will be returned only if `return_dict=True` is passed or that `config.return_dict` is set to `True`. This is no longer valid ever since the default configuration changed, thus it will be beneficial for the readers to update this portion to indicate that those values need to be `False` for a tuple to be returned.

This will likely save readers some time when adapting old code :)
Requesting review from @sgugger<|||||>Looks like you just need to run `make style` on your branch and we should be good to merge!<|||||>I have fixed the styling issues!<|||||>Thanks!