refactored grad scaler (#338)

colossalai/amp/naive_amp/grad_scaler/constant_grad_scaler.py

@@ -0,0 +1,16 @@
+#!/usr/bin/env python
+# -*- encoding: utf-8 -*-
+from .base_grad_scaler import BaseGradScaler
+
+__all__ = ['ConstantGradScaler']
+
+
+class ConstantGradScaler(BaseGradScaler):
+
+    def __init__(self, initial_scale: int, verbose: bool):
+        super().__init__(initial_scale, verbose)
+        self.log(f"Constant Gradient Scaler is initialized with scale {self.scale}", ranks=[0])
+
+    def update(self, overflow: bool) -> None:
+        # do nothing to maintain the current scale value
+        pass