Add actual dtype to RuntimeError("mat1 and mat2 must have the same dtype")

[danielkallendorf]

🚀 The feature, motivation and pitch

I was doing some testing and get the error message RuntimeError("mat1 and mat2 must have the same dtype") from a linear layer. The problem is fixed, but I would have found the issue faster, if the error message told me that one mat was float32 and the other float64. I.e. RuntimeError("mat1 and mat2 must have the same dtype, but got torch.float32 and torch.float64").

Alternatives

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Additional context

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cc @albanD @mruberry @jbschlosser @walterddr @mikaylagawarecki