Add VariableTensorId, store it in TensorTypeSet (#25597)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25597
We now take advantage of the new bitset representation TensorTypeSet to store "Variable-ness" of a tensor directly in the dispatch key. We introduce a new thread local TensorTypeSet "excluded" and replace the previous thread local boolean with it; we no longer have to query `is_variable()` to do dispatch (I didn't delete `is_variable`, because there are still a lot of uses of it). The key change is in `dispatchTypeId`.
Knock-on effects:
* Because Variable is now a TensorTypeId, I can eliminate the out-of-line registration `registerVariableOp` for variables; instead, make the registrar take a TensorTypeId (instead of a Backend) and you just register under the Variable key.
* Tensors aren't really ever created with Variable information initialized correctly at the start; instead, a tensor "becomes" a Variable because we set its `autograd_meta_`. These setters now correctly setup invariants on the dispatch type set. The new invariant is that if `autograd_meta_ != nullptr`, then `type_set().has(TensorTypeId::VariableTensorId)`.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D17265919
Pulled By: ezyang
fbshipit-source-id: a90a7ed14f5cb1086137483ae3d0646fcd4c42d0
