Enable fp16 in downstream/runner.py

[GiscardBiamby]

Feature request

Is your feature request related to a problem? Please describe.
Downstream fine-tuning can run faster and use less GPU memory if fp16 is enabled in downstrean/runner.py. It is already implemented in upstream/runner.py.

It should not affect any existing code since fp16 is off by default in the downstream configs. Users would opt-in by setting runner.fp16: true in their downstream configs.

Describe the solution you'd like
Mimic the implementation of mixed precision training from upstream/runner.py in downstream/runner.py

Describe alternatives you've considered
None

Additional context
I've implemented locally so I'm happy to send a PR if you would like.

[leo19941227]

Hello!

Sure! This sounds excellent.
Could you send the PR? I can test and merge it today.
Thanks!

[AdamGradient]

I'm just curious. In your previous experiments, did you find this to have an evident impact on performance