[quant] creating quint4x2 dtype for quantized tensors (#44678)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44678
This is a prototype PR that introduces 4 bit qtensors. The new dtype added for this is c10::quint4x2
The underlying storage for this is still uint8_t, so we pack 2 4-bit values in a byte while quantizing it.
This change uses most of the existing scaffolding for qtensor storage. We allocate storage
based on the dtype before creating a new qtensor.
It also adds a dispatch mechanism for this dtype so we can use this to get the bitwidth, qmin and qmax info
while quantizing and packing the qtensor (when we add 2-bit qtensor)
Kernels that use this dtype should be aware of the packing format.
Test Plan:
Locally tested
```
x = torch.ones((100, 100), dtype=torch.float)
qx_8bit = torch.quantize_per_tensor(x, scale=1.0, zero_point=2, dtype=torch.quint8)
qx = torch.quantize_per_tensor(x, scale=1.0, zero_point=2, dtype=torch.quint4x2)
torch.save(x, "temp.p")
print('Size float (B):', os.path.getsize("temp.p"))
os.remove('temp.p')
torch.save(qx_8bit, "temp.p")
print('Size quantized 8bit(B):', os.path.getsize("temp.p"))
os.remove('temp.p')
torch.save(qx, "temp.p")
print('Size quantized 4bit(B):', os.path.getsize("temp.p"))
os.remove('temp.p')
```
Size float (B): 40760
Size quantized 8bit(B): 10808
Size quantized 4bit(B): 5816
Imported from OSS
Reviewed By: raghuramank100
Differential Revision: D23993134
fbshipit-source-id: 073bf262f9680416150ba78ed2d932032275946d
