Use non-strict mode by default

[kwen2501]

torch.export has strict mode and non-strict mode. For difference, please read Non-Strict Export.

This PR switches to non-strict mode by default. Improving tracing success rate (no Dynamo graph break).

[wconstab]

im not convinced.

i should be easy to convince since at the moment i'm fighting with tracer issue after tracer issue.

however, if the answer is "turn off safety so it doesn't throw errors" i'm not sure i want to use the tracer at all.