upgrade pytorch 1.8 to 2.1.md

1. comment #include <THC/THC.h>
   replace THCudaCheck with AT_CUDA_CHECK
   add #include <ATen/cuda/CUDAContext.h>
   
2. replace THCCeilDiv with at::ceil_div
   add #include <ATen/ceil_div.h>
   
3. Delete THCudaMalloc
   add #include <ATen/cuda/ThrustAllocator.h>
   

  //THCudaCheck(THCudaMalloc(state, (void**) &mask_dev,
  //                      boxes_num * col_blocks * sizeof(unsigned long long)));

  //mask_dev = (unsigned long long*) THCudaMalloc(state, boxes_num * col_blocks * sizeof(unsigned long long));
  
  ↓
mask_dev = (unsigned long long*) c10::cuda::CUDACachingAllocator::raw_alloc(boxes_num * col_blocks * sizeof(unsigned long long));

4. Delete THCudaFree

  //THCState *state = at::globalContext().lazyInitCUDA(); 
  //THCudaFree(state, mask_dev);
  
  ↓
  c10::cuda::CUDACachingAllocator::raw_delete(mask_dev);

5. Redefine AT_CHECK
   add these lines in the top of the file(*.cu,*.cpp,*.cuh)
   

#ifndef AT_CHECK
#define AT_CHECK TORCH_CHECK
#endif

6. replace <THC/THCDeviceUtils.cuh> with <ATen/cuda/DeviceUtils.cuh>