18_callback.fp16: Create Docments

[marii-moe]

Add documents to 18_callback.fp16.

See the style guide and contribution guide for more details (links will be added when created).

This requires a moderate familiarity with fastai, and an understanding of how computers represent floating point numbers. This notebook defines how we do mixed precision in fastai.

Documents progress(only one section:

• [ ] MixedPrecision
• [ ] Creating the master copy of the parameters
• [ ] Copy the gradients from model params to master params
• [ ] Copy the master params to the model params
• [ ] Checking for overflow
• [ ] NonNativeMixedPrecision

[marii-moe]

Currently working on this one.