fix: assertion and chain-of-thought

[Shangyint]

Previously, ChainOfThought does not work well with assertions due to an error in handling the rationale field of the CoT module. This pr fixes this issue. A future improvement would be implementing unit tests for CoT and Retry in test_retry.py.

[okhat]

Thank you @Shangyint !! Looks like we had tests that are tied to the old behavior. Do we trmove/efit them?

[Shangyint]

Yes, I figured out the same thing. Pushed a fix!

[chiragshah285]

hey all @Shangyint the assertion fix broke a few things. I'm unable to use dspy.Suggest(), here is a minimal working example of an error: AttributeError: 'Predict' object has no attribute 'new_signature'. Did you mean: 'signature'?

https://gist.github.com/chiragshah285/cc1756cde35b8cf14aa04282f3f744e1

line 260 in assertions.py, i've patched it with this but not sure if this is the right fix:

                            if hasattr(error_state[0], 'new_signature'):
                                output_fields = error_state[0].new_signature.output_fields
                            else:
                                output_fields = error_state[0].signature.output_fields

[Shangyint]

Hey @chiragshah285, thank you! I submitted your patch as #1372.