Add python `nested_tensor` and `as_nested_tensor` constructors in `torch.nested` (#85593)
Remove `torch.nested_tensor` which has erroneous behavior wrt gradients (could be either leaf or not leaf). Introduce `torch.nested.nested_tensor` and `torch.nested.as_nested_tensor` in the vein of `torch.tensor` and `torch.as_tensor`. Done in nested `__init__.py` for now but can move to pybind in future (when we want to load from numpy/nested lists ).
Discussed offline with @cpuhrsch and pybind constructor (https://github.com/pytorch/pytorch/pull/85536) was more gnarly than expected, so we can move to that when we do need loading from numpy etc.
Differential Revision: [D39806622](https://our.internmc.facebook.com/intern/diff/D39806622)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85593
Approved by: https://github.com/drisspg, https://github.com/cpuhrsch
