Port symeig to ATen and enable batching of inputs (#21858)
Summary:
Changelog:
- Port `symeig` from TH/THC to ATen
- Enable batching of matrix inputs for `symeig`
- Modify derivative computation based on batching
- Update docs to reflect the change
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21858
Test Plan: - Added additional tests in `test_torch.py` (with a port to `test_cuda.py`) and `common_methods_invocations.py` to test if both the port and batching work.
Differential Revision: D15981789
Pulled By: soumith
fbshipit-source-id: ab9af8361f8608db42318aabc8421bd99a1ca7ae
