Add Pluggable CUDA allocator backend (#86786)
Fixes #43144
This uses the Backend system added by [82682](https://github.com/pytorch/pytorch/pull/82682) to change allocators dynamically during the code execution. This will allow us to use RMM, use CUDA managed memory for some portions of the code that do not fit in GPU memory. Write static memory allocators to reduce fragmentation while training models and improve interoperability with external DL compilers/libraries.
For example, we could have the following allocator in c++
```c++
#include <sys/types.h>
#include <cuda_runtime_api.h>
#include <iostream>
extern "C" {
void* my_malloc(ssize_t size, int device, cudaStream_t stream) {
void *ptr;
std::cout<<"alloc "<< size<<std::endl;
cudaMalloc(&ptr, size);
return ptr;
}
void my_free(void* ptr) {
std::cout<<"free "<<std::endl;
cudaFree(ptr);
}
}
```
Compile it as a shared library
```
nvcc allocator.cc -o alloc.so -shared --compiler-options '-fPIC'
```
And use it from PyTorch as follows
```python
import torch
# Init caching
# b = torch.zeros(10, device='cuda')
new_alloc = torch.cuda.memory.CUDAPluggableAllocator('alloc.so', 'my_malloc', 'my_free')
old = torch.cuda.memory.get_current_allocator()
torch.cuda.memory.change_current_allocator(new_alloc)
b = torch.zeros(10, device='cuda')
# This will error since the current allocator was already instantiated
torch.cuda.memory.change_current_allocator(old)
```
Things to discuss
- How to test this, needs compiling external code ...
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86786
Approved by: https://github.com/albanD
