[op] Autograd PrivateUse1 stubs

[samthakur587]

Parent: #15

Op / kernel

Autograd PrivateUse1 stubs

Decide and document autograd strategy; stub or implement AutogradPrivateUse1.

Acceptance criteria

• ATen / custom op registered on PrivateUse1
• Host (CPU sim) implementation
• OpenCL kernel + runtime launch (or explicit deferral note)
• Test vs CPU reference in tests/
• Linked PR closes this issue

Category

cat-infra