set fp16 to false if bf16, update bf16: auto in example YAMLs

[winglian]

#1116 enables bf16: auto support, which should simplify whether an end user needs to think about using bf16 for their particular GPU. This updates all the example yaml files to use this.

[NanoCode012]

Cool! I'm a bit conflicted on the naming choice of using bf16: auto instead of some autocast: true, but it works

[winglian]

@NanoCode012 perhaps we can discuss having something like autocast: fp8,bf16,fp16 which would attempt to see if the gpu supports any of those in that order and fallback to fp32, whereas autocast: bf16 would only check for bf16 and skip fp16 for fp32?
For now, let's keep this and have a discussion about the above proposal?