[Hotfix] Fix Swin model outputs
# What does this PR do?

This PR:

- fixes some critical outputs of `SwinModel`, to be exactly as `ViTModel`, `BeitModel`, `DeitModel`. More specifically, the `last_hidden_state` previously returned a pooled output, which shouldn't be the case. Instead, the output should be a tensor of shape (batch_size, seq_len, hidden_size), and the pooler output a tensor of shape (batch_size, hidden_size).
- adds `problem_type` support to `SwinForImageClassification`
- fixes the integration test

To do:

- [x] more of a nit, but the pooler in Swin is called `self.pool` rather than `self.pooler` (as in models like BERT, ViT). We could rename it to `self.pooler` for consistency.
_The documentation is not available anymore as the PR was closed or merged._