Support `torch.save` for saving values during execution (#18154)
Summary:
This PR makes `torch.save` call out to the pickler which saves a tensor in the same format that `torch.save()` does, the file looks like `| pickle archive 1 (includes sizes, strides, requires_grad, etc...) | pickle archive 2 (list of tensor keys) | tensor binary data |` and can be read back in with `torch.load(my_file, pickle_module=torch.jit._pickle)`
Fixes #18003
Unpickling in the JIT for things such as model parallelism will be a follow up PR
](https://our.intern.facebook.com/intern/diff/15015160/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18154
Pulled By: driazati
Differential Revision: D15015160
fbshipit-source-id: ef76a44b8c243f4794cd7e245ec8305e965bc59f
