better support set_default_dtype(torch.float16), update doc (#121730)
1. Fixes #121300
2. Previously, calling `torch.tensor([2j])` after `torch.set_default_dtype(torch.float16)` will cause a runtime error. This PR also fixes it and enables test.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121730
Approved by: https://github.com/peterbell10
