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Device conversion with to() and from_numpy() and numpy() in PyTorch

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7/16/2024

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Device conversion with to() and from_numpy() and numpy() in PyTorch

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*My post explains how to create and acceess a tensor.

to() can do device conversion as shown below:

*Memos:

to() can be used with a tensor but not with torch.
The 1st argument with a tensor is device(Optional-Defalut:None-Type:str, int or device()): *Memos:
- If it's None, the device of a tensor is not converted.
- cpu, cuda, ipu, xpu, mkldnn, opengl, opencl, ideep, hip, ve, fpga, ort, xla, lazy, vulkan, mps, meta, hpu, mtia or privateuseone can be set to device.
- Setting 0 to device uses cuda(GPU). *The number must be zero or positive.
- My post explains device().
A copied tensor can be created.

import torch

cpu_tensor = torch.tensor([0, 1, 2])

cpu_tensor.device
# device(type='cpu')

cpu_tensor.to().device
# device(type='cpu')

gpu_tensor = cpu_tensor.to(device='cuda:0')
gpu_tensor = cpu_tensor.to(device='cuda')
gpu_tensor = cpu_tensor.to(device=0)
gpu_tensor = cpu_tensor.to(device=torch.device(device='cuda:0'))
gpu_tensor = cpu_tensor.to(device=torch.device(device='cuda'))
gpu_tensor = cpu_tensor.to(device=torch.device(device=0))
gpu_tensor = cpu_tensor.to(device=torch.device(type='cuda', index=0))
gpu_tensor = cpu_tensor.to(device=torch.device(type='cuda'))

gpu_tensor.device
# device(type='cuda', index=0)

gpu_tensor.to().device
# device(type='cuda', index=0)

cpu_tensor = gpu_tensor.to(device='cpu')

cpu_tensor.device
# device(type='cpu')

cuda() and cpu() can change the device of a tensor to GPU(CUDA) and CPU respectively as shwon below:

*Memos:

cuda() or cpu() can be used with a tensor but not with torch.
For cuda(), the 1st argument with a tensor is device(Optional-Default:None-Type:str, int or device()). *Memos:
- The default is the current GPU(CUDA).
- cpu, cuda, ipu, xpu, mkldnn, opengl, opencl, ideep, hip, ve, fpga, ort, xla, lazy, vulkan, mps, meta, hpu, mtia or privateuseone can be set to device.
- Setting 0 to device uses cuda(GPU). *The number must be zero or positive.
- My post explains device().
A copied tensor is created.

import torch

cpu_tensor = torch.tensor([0, 1, 2])

cpu_tensor.device
# device(type='cpu')

gpu_tensor = cpu_tensor.cuda()
gpu_tensor = cpu_tensor.cuda(device='cuda:0')
gpu_tensor = cpu_tensor.cuda(device='cuda')
gpu_tensor = cpu_tensor.cuda(device=0)
gpu_tensor = cpu_tensor.cuda(device=torch.device(device='cuda:0'))
gpu_tensor = cpu_tensor.cuda(device=torch.device(device='cuda'))
gpu_tensor = cpu_tensor.cuda(device=torch.device(device=0))
gpu_tensor = cpu_tensor.cuda(device=torch.device(type='cuda', index=0))
gpu_tensor = cpu_tensor.cuda(device=torch.device(type='cuda'))

gpu_tensor.device
# device(type='cuda', index=0)

cpu_tensor = gpu_tensor.cpu()

cpu_tensor.device
# device(type='cpu')

from_numpy() can convert a NumPy array to a PyTorch tensor as shown below:

*Memos:

from_numpy() can be used with torch but not with a tensor.
The 1st argument with torch(Required-Type:ndarray). *There is no keyword argument.
The type of a NumPy array is also inherited to a PyTorch tensor.

import torch

my_array = np.array([0., 1., 2.])

my_array.dtype
# dtype('float64')

my_tensor = torch.from_numpy(my_array)

my_tensor
# tensor([0., 1., 2.], dtype=torch.float64)

numpy() can convert a PyTorch tensor to a NumPy array as shown below:

*Memos:

numpy() can be used with a tensor but not with torch.
There is force argument with a tensor(Optional-Default:False-Type:bool). *Memos:
- If it's True, a GPU(CUDA) PyTorch tensor can be converted to a NumPy array which may be a copy.
- force= must be used.
The type of a PyTorch tensor is also inherited to a NumPy array.

import torch

my_tensor = torch.tensor([0., 1., 2.])

my_tensor.dtype
# torch.float32

my_array = my_tensor.numpy()

my_array
# array([0., 1., 2.], dtype=float32)

my_tensor = torch.tensor([0., 1., 2.], device='cuda:0')

my_tensor.numpy(force=True)
# array([0., 1., 2.], dtype=float32)

my_tensor = torch.tensor([0., 1., 2.], device='cuda:0')

my_tensor.numpy()
my_tensor.numpy(force=False)
# Error