[amp] Add naive amp demo (#3774)

* [mixed_precison] add naive amp demo

* [mixed_precison] add naive amp demo

colossalai/booster/mixed_precision/__init__.py

 from .bf16 import BF16MixedPrecision
 from .fp8 import FP8MixedPrecision
 from .fp16_apex import FP16ApexMixedPrecision
+from .fp16_naive import FP16NaiveMixedPrecision
 from .fp16_torch import FP16TorchMixedPrecision
 from .mixed_precision_base import MixedPrecision
 
 __all__ = [
     'MixedPrecision', 'mixed_precision_factory', 'FP16_Apex_MixedPrecision', 'FP16_Torch_MixedPrecision',
-    'FP32_MixedPrecision', 'BF16_MixedPrecision', 'FP8_MixedPrecision'
+    'FP32_MixedPrecision', 'BF16_MixedPrecision', 'FP8_MixedPrecision', 'FP16NaiveMixedPrecision'
 ]
 
 _mixed_precision_mapping = {
     'fp16': FP16TorchMixedPrecision,
     'fp16_apex': FP16ApexMixedPrecision,
+    'fp16_naive': FP16NaiveMixedPrecision,
     'bf16': BF16MixedPrecision,
     'fp8': FP8MixedPrecision
 }

colossalai/booster/mixed_precision/fp16_naive.py

+from .mixed_precision_base import MixedPrecision
+
+
+class FP16NaiveMixedPrecision(MixedPrecision):
+    pass