fix(dspy): fix save/load for predict method

[GFarnon]

Allow the dump_state/load_state methods in dspy.Predict to include prefix and desc for all signature fields

[okhat]

Thank you so much @GFarnon ! If I understand correctly, this will not be backwards compatible with existing saved files?

[okhat]

Also looks like the ruff check is failing but that's more minor.

[GFarnon]

Will add a fallback & fix

[arnavsinghvi11]

Hi @GFarnon , thanks for the PR! the ruff fix will be fixed once you merge with main.

@okhat I've tested the change with the intro.ipynb and it doesn't have any breaking changes - although it does not rely on save/load functionalities. Would the CoT.ipynb be better to test this (although that does not have cache currently so it wouldn't break any repo-wide changes).

@GFarnon would this change impact ChainOfThought as well, given that there are extended_... fields added to the state? For context, ChainOfThought inherits from Predict but reconstructs the signature within its internals.