AnyPrecision optimizer dynamic casting

[atturaioe]

Describe the bug: exp_avg_sq.mul_(beta2).addcmul_(grad, grad, value=1 - beta2) dtypes of exp_avg_sq and grad differ while using in-place operations.

Describe how to reproduce:

# uses default hyperparameters such as momentum=float32 and variance=bfloat16
optim = AnyPrecisionAdamW(model.parameters())
optim.step()

Error log RuntimeError: !needs_dynamic_casting<func_t>::check(iter) INTERNAL ASSERT FAILED at "../aten/src/ATen/native/cpu/Loops.h":347, please report a bug to PyTorch.