fix assert message (#1040)
* fix assert
The current assert "Model must initialized in fp16 mode for ZeRO Stage 3." needs TLC - I rewrote it completely to match its cousen assert, so now we have 2 consistent matching asserts:
- f"fp16 is enabled but one or several model parameters have dtype that is not fp16"
- f"fp16 is not enabled but one or several model parameters have dtype of fp16"
* remove f
