API to query whether a field needs gradients

[ifsheldon]

Concisely describe the proposed feature
I would like to request an API to query whether a field needs gradients.

Describe the solution you'd like (if any)
An attribute or property attached to a field object, which states whether a field needs gradients in backward-mode differentiation.

Additional comments
This feature request dates back to #2498 whose conversation was in the old Slack community channel. In my use case, I need it to check whether a field passed by the user of my library stannum needs gradients or not, and based on that my library creates different tensors. Besides my use case, it is a common need to check if a field needs gradients or not.

[erizmr]

Hi @ifsheldon , we are implementing this feature and want to double check: if the API you request is to check whether a field is annotated with needs_grad (but may has been or has not been allocated with a grad field) or to check whether the field has been allocated a grad field?

[ifsheldon]

Yeah, it's sufficient I think. As the name needs_grad suggests, a field needs gradients may not have been allocated with gradients, which is fine.