pin_memory malloc now uses existing context if available. (#22229)
Summary:
This is achieved by using `cuDevicePrimaryCtxGetState` as a way to check whether a primary context exists on a device. It is not too slow, from this benchmark of a single call to it on CUDA 10.1, Titan Xp, driver 415.27:
```
---------------------------------------------------------------------
Benchmark Time CPU Iterations
---------------------------------------------------------------------
BM_cuDevicePrimaryCtxGetState 301 ns 301 ns 2319746
```
Commits:
1. Add `CUDAHooks::getDeviceWithPrimaryContext` which returns a device index with primary context (if exists).
Link `c10/cuda` against `libcuda` for device API calls.
2. Use `getDeviceWithPrimaryContext` to check primary context in `pin_memory`.
Fix `OptionalDeviceGuard` doc.
3. Refactor `test_cuda_primary_ctx.py` to support multiple tests.
Add test for this in that file.
Fixes https://github.com/pytorch/pytorch/issues/21081.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22229
Differential Revision: D16170194
Pulled By: zou3519
fbshipit-source-id: 485a45f211b7844c9e69c63f3b3b75194a796c5d
