Einsum is not added to the supported list for autocast

[avizon-aws]

We noticed that einsum is not added to the supported ops list for low precision policy in autocast, is there a reason for that? Does this op have some issues in the support?

[miladm]

cc @tengyifei to help with this issue

[aws-nm9]

In order to support einsum, would it require more extensive changes or just a change to torch_xla/csrc/autocast_mode.cpp that adds KERNEL_XLA(einsum, lower_precision_fp)?

[aws-nm9]

PR created for einsum support in autocast: #8420. An extension of the same is #8421.