add mixed precision support to deepxde

[g-w1]

No description provided.

[lululxvi]

If I use the API here to set mixed precision, then all the demo code can run in mixed precision?

[g-w1]

Not all. The L-BFGS optimizer doesn't work in mixed precision. But if you add the line dde.config.set_default_float("mixed") to the top of your file, it should work unless it uses a feature (like L-BFGS) that is not supported with mixed precision. I have tested this on the Burgers.py example and it works.

[g-w1]

Sorry for the late reply, hopefully I've addressed some of the issues