vmap: temporarily disable support for random functions (#42617)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42617
While we figure out the random plan, I want to initially disable
support for random operations. This is because there is an ambiguity in
what randomness means. For example,
```
tensor = torch.zeros(B0, 1)
vmap(lambda t: t.normal_())(tensor)
```
in the above example, should tensor[0] and tensor[1] be equal (i.e.,
use the same random seed), or should they be different?
The mechanism for disabling random support is as follows:
- We add a new dispatch key called VmapMode
- Whenever we're inside vmap, we enable VmapMode for all tensors.
This is done via at::VmapMode::increment_nesting and
at::VmapMode::decrement_nesting.
- DispatchKey::VmapMode's fallback kernel is the fallthrough kernel.
- We register kernels that raise errors for all random functions on
DispatchKey::VmapMode. This way, whenever someone calls a random
function on any tensor (not just BatchedTensors) inside of a vmap block,
an error gets thrown.
Test Plan: - pytest test/test_vmap.py -v -k "Operators"
Reviewed By: ezyang
Differential Revision: D22954840
Pulled By: zou3519
fbshipit-source-id: cb8d71062d4087e10cbf408f74b1a9dff81a226d
