Guard weight_dequant.args[1] access in _quantize_fused_conv_bias pass

[ethansfng]

Summary:
For models whose conv weight isn't wrapped in a dequantize_per_tensor/per_channel call weight_dequant.args has fewer than 2 elements and the access throws IndexError.

Adds a guard to prevent this error.

Differential Revision: D105503056

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