[dynamo] Fix import if numpy is not installed (#105711)
This [line](https://github.com/pytorch/pytorch/blob/main/torch/_dynamo/allowed_functions.py#L18) results in an import issue if numpy is not installed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105711
Approved by: https://github.com/yanboliang, https://github.com/ezyang
