allow to override the checkpoint's droupout settings

[l-k-11235]

This PR provides the ability to override the checkpoint's dropout settings (dropout, attention_dropout) when finetuning a Transformer model.
A new flag override_opts is added. This overriding can thus be disabled with --override_opts False
It is not possible at the moment because the checkpoint's model_opts override those set in the config.
See the following discussion about this topic:
https://forum.opennmt.net/t/tranformer-change-dropout-when-finetuning/4625

[vince62s]

@l-k-11235 @francoishernandez is this ok to merge ?

[francoishernandez]

LGTM

[vince62s]

I am just wondering if model_opt = opt is not too strong.
what if some options are missing in opt ? shoudn't we copy only the ones that are modified?

[francoishernandez]

At this stage of the code opt should be a valid opts object.