Support NVFP4 fp32 bias #3525
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Support NVFP4 fp32 bias #3525
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**Summary:** Today we hit this error with fp32 inputs + bias: ``` RuntimeError: Bias is not supported when module weight is in fp32 (out_dtype=Float32). Please use bfloat16 or float16 weights, or remove the bias from the linear layer. ``` This is thrown by `NVFP4DynamicActivationNVFP4WeightConfig` but it's trying to guard against this underlying `_scaled_mm` error: ``` RuntimeError: Bias is not supported when out_dtype is set to Float32 ``` This commit works around these errors by adding the bias separately in this case, similar to what float8 does. **Test Plan:** ``` pytest test/prototype/mx_formats/test_inference_workflow.py -k test_inference_workflow_nvfp4 ```
🔗 Helpful Links🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/3525
Note: Links to docs will display an error until the docs builds have been completed. ✅ You can merge normally! (1 Unrelated Failure)As of commit 83c92e3 with merge base 486fe0d ( BROKEN TRUNK - The following job failed but were present on the merge base:👉 Rebase onto the `viable/strict` branch to avoid these failures
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meta-cla bot added the CLA Signed 
andrewor14 added the topic: improvement 
drisspg reviewed Dec 22, 2025
| # since bias is not quantized | ||
| should_add_bias_separately = (scale_result is not None) and (bias is not None) | ||
| # | ||
| # (2) RuntimeError: Bias is not supported when out_dtype is set to Float32 |
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Okay this is what I thought would happen
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Yeah but this only happens if per_tensor_scale=None (by default it is not), so users generally won't run into the _scaled_mm error. Either way this PR fixes that case
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can you just create a note somewhere on the conversions / casting path for the paths?
drisspg approved these changes Dec 22, 2025
vkuzo approved these changes Dec 22, 2025
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Summary: Today we hit this error with fp32 inputs + bias:
This is thrown by
NVFP4DynamicActivationNVFP4WeightConfigbut it's trying to guard against this underlying_scaled_mmerror:This commit works around these errors by adding the bias separately in this case, similar to what float8 does.
Test Plan: