Relax QAT dtype assertion (#692)

This was added originally for perf reasons specific to 8da4w,
but the autograd.Function has since been adapted for more general
use. A few users are hitting this assertion error.

More context: pytorch/torchtune#1333

test/quantization/test_qat.py

@@ -422,9 +422,6 @@ def test_qat_4w_primitives(self):
 
     @unittest.skipIf(not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower")
     @unittest.skipIf(not _CUDA_IS_AVAILABLE, "skipping when cuda is not available")
-    # TODO: remove once we fix int4 error: https://github.com/pytorch/ao/pull/517
-    @unittest.skipIf(TORCH_VERSION_AT_LEAST_2_4, "assert input.dtype == torch.float32" )
-    @unittest.skipIf(TORCH_VERSION_AT_LEAST_2_5, "int4 doesn't work for 2.5+ right now")
     def test_qat_4w_linear(self):
         from torchao.quantization.prototype.qat.api import Int4WeightOnlyQATLinear
         from torchao.quantization.GPTQ import WeightOnlyInt4Linear
@@ -453,9 +450,6 @@ def test_qat_4w_linear(self):
 
     @unittest.skipIf(not TORCH_VERSION_AT_LEAST_2_4, "skipping when torch version is 2.4 or lower")
     @unittest.skipIf(not _CUDA_IS_AVAILABLE, "skipping when cuda is not available")
-    # TODO: remove once we fix int4 error: https://github.com/pytorch/ao/pull/517
-    @unittest.skipIf(TORCH_VERSION_AT_LEAST_2_4, "assert input.dtype == torch.float32" )
-    @unittest.skipIf(TORCH_VERSION_AT_LEAST_2_5, "int4 doesn't work for 2.5+ right now")
     def test_qat_4w_quantizer(self):
         from torchao.quantization.prototype.qat import Int4WeightOnlyQATQuantizer
         from torchao.quantization.GPTQ import Int4WeightOnlyQuantizer

torchao/quantization/prototype/qat/utils.py

@@ -36,13 +36,6 @@ def forward(
         block_size: List[int],
         zero_point_domain: ZeroPointDomain = ZeroPointDomain.INT,
     ) -> torch.Tensor:
-        # Note: for bf16 inputs, casting them to fp32 has the unexpected
-        # side effect of reducing memory footprint significantly, presumably
-        # because bf16 * fp32 kernels are not as memory efficient
-        assert input.dtype == torch.float32
-        assert scales.dtype == torch.float32
-        assert zero_points.dtype == torch.int32
-
         (fq, mask) = fake_quantize_affine_cachemask(
             input,
             block_size,