Implements torch.isclose for complex tensors (#36456)
Summary:
Previously torch.isclose would RuntimeError when called on complex tensors. This update updates torch.isclose to run on complex tensors and be consistent with [NumPy](https://numpy.org/doc/1.18/reference/generated/numpy.isclose.html). However, NumPy's handling of NaN, -inf, and inf values is odd, so I adopted Python's [cmath.isclose](https://docs.python.org/3/library/cmath.html) behavior when dealing with them. See https://github.com/numpy/numpy/issues/15959 for more on NumPy's behavior.
While implementing complex isclose I also simplified the isclose algorithm to:
- A is close to B if A and B are equal, if equal_nan is true then NaN is equal to NaN
- If A and B are finite, then A is close to B if `abs(a - b) <= (atol + abs(rtol * b))`
This PR also documents torch.isclose, since it was undocumented, and adds multiple tests for its behavior to test_torch.py since it had no dedicated tests.
The PR leaves equal_nan=True with complex inputs an error for now, pending the outcome of https://github.com/numpy/numpy/issues/15959.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36456
Differential Revision: D21159853
Pulled By: mruberry
fbshipit-source-id: fb18fa7048e6104cc24f5ce308fdfb0ba5e4bb30
