support nested_tensor * scalar (#80284)
In transformer, the scale step in attention has a `nested_tensor / scalar` operation. There are two ways to support that:
1. directly support `nested_tensor / scalar`:
* pro: straightforward, good UX
* con: is dispatching `mul(nested tensor, regular tensor)` a good practice?
2. let user manually convert `scalar` to `nested_scalar = torch.nested_tensor([broadcast_scalar])`
* pro: dispatcher only has to deal with `mul(nested tensor, nested tensor)`
* con: confusing manual conversions, bad UX
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80284
Approved by: https://github.com/cpuhrsch
