Do not raise error when there is mixed dtype in the checkpoint (#6247)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6247

This warning message got turned into an exception during the refactor of D64145852. It fails our export flow for llama 3.2 1B/3B. This diff fixes it.

Reviewed By: mergennachin, dvorjackz

Differential Revision: D64438373

fbshipit-source-id: 3dbfe44b95c94111de8cb2773ee9e642f9b896e6