[Core ATen Opset] Lower aten_tanh

[wonjoo-wj]

In order for PyTorch/XLA to support the PyTorch core ATen opset, it requires lowering each core ATen op in PyTorch/XLA. This issue is used to track the PyTorch/XLA lowering for aten_tanh.

Here are some general guidelines to lowering this op:

• Uncomment @unittest.skip or @unittest.expectFailure and run the unit test at test_core_aten_ops.py. Eg: pytest test/test_core_aten_ops.py -k test_aten_tanh_0
• Make code changes until the test passes. Read and follow fix_lowering_for_core_aten_ops.md for ideas to fix.
  • There may be multiple unit tests for a single op. For this op, the corresponding unit tests are:
    • test_aten_tanh_0
    • test_aten_tanh_1
    • test_aten_tanh_2
  • Please also uncomment the skips for all these tests and ensure all tests are fixed.
  • Note that sometimes the fix may be to fix the unit tests itself. Please take a look at the corresponding unit tests to make sure the tests are valid.
• Submit the PR!

For any questions, feel free to leave a comment in this PR.