Sets CUDA_MODULE_LOADING to LAZY when not set by the user (#85692)
This PR sets CUDA_MODULE_LOADING if it's not set by the user. By default, it sets it to "LAZY".
It was tested using the following commands:
```
python -c "import torch; tensor=torch.randn(20, 16, 50, 100).cuda(); free, total = torch.cuda.cudart().cudaMemGetInfo(0); print(total-free)"
```
which shows a memory usage of: 287,047,680 bytes
vs
```
CUDA_MODULE_LOADING="DEFAULT" python -c "import torch; tensor=torch.randn(20, 16, 50, 100).cuda(); free, total = torch.cuda.cudart().cudaMemGetInfo(0); print(total-free)"
```
which shows 666,632,192 bytes.
C++ implementation is needed for the libtorch users (otherwise it could have been a pure python functionality).
cc: @ptrblck @ngimel @malfet
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85692
Approved by: https://github.com/malfet
