fit-params : Handle n_ctx 0 for models that entirely fit with n_ctx_train

[65a]

During initial fitting, if the n_ctx is 0, try to fit using n_ctx_train.
If initial fitting was successful, make sure to set n_ctx to n_ctx_train instead of leaving it at zero.

This could be done differently, but this approach avoids making cparams mutable deeper in the stack, and then having to clean it back to 0 if we aren't going to early return.

Fixes #18889