Support for torch 1.9.0
This PR adds support for torch 1.9.0. It upgrades the CPU CI to use torch 1.9.0, and the GPU CI to use PyTorch's 1.9.0 docker image to run tests.

As discussed with @michaelbenayoun, this puts a hard requirement on having a specific torch version for torch fx to be run. The idea is that:
- The torch fx support in `transformers` is currently experimental, and will be updated *without* backwards compatibility requirements
- To that end, it should always support the latest PyTorch version and not the earlier ones.
- However PyTorch 1.9.0 will not be supported due to https://github.com/pytorch/pytorch/pull/59569
- To that end, we setup a specific version requirement on `torch` in order to offer torch FX support.

Running on torch 1.8.0 and torch 1.8.1, as well as the various torch 1.8.1-cu111 and other 1.8.x versions works correctly.
Running on torch < 1.8 or torch > 1.8 returns:

```
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/opt/pycharm-professional/plugins/python/helpers/pydev/_pydev_bundle/pydev_umd.py", line 197, in runfile
    pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
  File "/opt/pycharm-professional/plugins/python/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "/home/lysandre/.config/JetBrains/PyCharm2021.1/scratches/torchfx.py", line 6, in <module>
    traced_model = symbolic_trace(
  File "/home/lysandre/transformers/src/transformers/modeling_fx_utils.py", line 374, in symbolic_trace
    tracer = HFTracer(batch_size=batch_size, sequence_length=sequence_length, num_choices=num_choices)
  File "/home/lysandre/transformers/src/transformers/modeling_fx_utils.py", line 152, in __init__
    raise ImportError(
ImportError: Found an incompatible version of torch. Found version 1.9.0, but only version 1.8 is supported.
```
Shouldn't the fx tests be skipped correspondingly? I see the CI logs show that they all passed with 1.9.0 - how is that possible?<|||||>The `is_torch_fx_available` returns `False` as the versions aren't compatible. The tests for torch.fx require `is_torch_fx_available` to be `True` in order to run!

Yes, switching back to > 1.9.0 once the issue is fixed works for me.<|||||>OK, but the tests were reported as passed and not skipped. So another todo for down the road is to add a skip rule, so that we don't get a misleading report and have a skipped test appearing as passed. Don't have to do it now.<|||||>They should have a decorator for that, rather than the in-test check. Would be better reported as skipped, indeed!