Fix LongformerModel hidden states
# What does this PR do?

`LongformerModel` currently returns `hidden_states` that is padded, while `TFLongformerModel` and `LEDEncoder` return un-padded version.

This PR makes `LongformerModel` return the un-padded `hidden_states` .

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`LEDEncoder` also returns  un-padded `attentions`. Maybe the same logic should be applied to `LongformerModel` too.
_The documentation is not available anymore as the PR was closed or merged._<|||||>The test needs to be adjusted due to the change. But I would like to have some feedback first before proceeding.<|||||>@patrickvonplaten could you take a look here ?<|||||>Thanks for reviewing! I will finalize this PR by adjusting the test.<|||||>@patrickvonplaten @patil-suraj 

(update)
- unpad `attentions` too for both `LongformerModel` / `TFLongformerModel`. (see below for the reasons)
- move unpadding logic in `(TF)LongformerModel` to `(TF)LongformerEncoder`
- update the tests

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# unpad `attentions`

Current  `LongformerModel` / `TFLongformerModel` don't unpad `attentions`. I am going to unpad the `attentions` just like what have been done for `LEDModel` & `TFLEDModel`. 

This is required to make `TFLongformerModelTest` pass (in an easy way).

(I don't think there is particular reason not to unpad `attentions` in `LongformerModel` / `TFLongformerModel`, but want to make sure)

## More details

In `LEDModel` & `TFLEDModel`, the `attentions` are also un-padded:

https://github.com/huggingface/transformers/blob/57882177becb85560f1ff931abb1b0b75d67e70d/src/transformers/models/led/modeling_led.py#L1871-L1872

https://github.com/huggingface/transformers/blob/57882177becb85560f1ff931abb1b0b75d67e70d/src/transformers/models/led/modeling_tf_led.py#L1814-L1819



However, `LongformerModel` / `TFLongformerModel` don't perform unpadding for `attentions` (but do unpadding for `hidden_states`). This causes a problem for `TFLongformerModelTester`:

https://github.com/huggingface/transformers/blob/57882177becb85560f1ff931abb1b0b75d67e70d/tests/test_modeling_tf_longformer.py#L77-L81

The testings for both `attentions` and `hidden_states` are controlled by `self.encoder_seq_length` => so we need to unpad `attentions` too (because `hidden_states` are unpadded).

