bitsandbytes.functional.quantize_4bit, bitsandbytes.functional.quantize_blockwise require contiguous input, fail silently if not #1690
Description
System Info
Python 3.12.3, on an AWS P4 instance.
Reproduction
Say I call quantize_4bit with x of shape (a, b, c, blocksize). Call num_ch = a * b * c. I am getting qx, state, so that:
state.absmax.shape = (num_ch,)qx.shape = (num_ch * blocksize // 2,),qx.dtype = torch.uint8
My best guess was the memory layout of qx and state.absmax allows me to do qx.view(a, b, c, blocksize // 2) and state.absmax.view(a, b, c) and then work with that. But this does not seem to work.
# x.shape = (4, 3, 38, 256)
qx, state = quantize_4bit(x, blocksize=256)
absmax_cmp = x.view(-1, 256).abs().max(dim=-1)
torch.testing.assert_close(state.absmax, absmax_cmp)
start, end = 10, 15
partx = x[:, :, start:end, :]
qx_part, state_part = quantize_4bit(partx, blocksize=256)
full = state.absmax.view(4, 3, 38)
part = state_part.absmax(4, 3, 5)
torch.testing.assert_close(full[:, :, start:end], part)
full = qx.view(4, 3, 38, -1)
part = qx_part.view(4, 3, 5, -1)
torch.testing.assert_close(full[:, :, start:end], part)As it stands, this fails with the 2nd assert. But if I replace:
partx = x[:, :, start:end, :].contiguous()
Then, it all works. This means quantize_4bit does not work properly if its input is not contiguous. But it also does not check that, so things just fail silently.
Expected behavior
I'd expect quantize_4bit and also quantize_blockwise either to check whether the input is contiguous, or to convert it to contiguous before quantization.
If I find some time, I submit a PR, which should also include a test that the example above works.