[ `ForSequenceClassification`] Support `left` padding
# What does this PR do?
Update the computation of `sequence_lengths` that determines the index of the outpus to be pooled on models that use pooled inputs in such a way.
Adresses #24265

- [ ] add a common test to make sure whether you pad left or right, sequence outputs are the same
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24979). All of your documentation changes will be reflected on that endpoint.<|||||>Test seems flaky:
```python 
FAILED tests/models/gptj/test_modeling_gptj.py::GPTJModelTest::test_pt_tf_model_equivalence - AssertionError: 0.14537059 not less than or equal to 1e-05 : outputs.logits: Difference between PyTorch and TF is 0.14537058770656586 (>= 1e-05).
FAILED tests/models/gptj/test_modeling_tf_gptj.py::TFGPTJModelTest::test_pt_tf_model_equivalence - AssertionError: 0.21959408 not less than or equal to 1e-05 : outputs.logits: Difference between torch and tf is 0.2195940762758255 (>= 1e-05).
```