fix some cast error. (#26884)

python/paddle/fluid/contrib/mixed_precision/fp16_utils.py

@@ -74,7 +74,7 @@ def _insert_cast_op(block, op, idx, src_dtype, dest_dtype):
                 continue
         for in_var_name in op.input(in_name):
             in_var = block.var(in_var_name)
-            if in_var.type not in valid_types:
+            if in_var.type not in valid_types or in_var.dtype == dest_dtype:
                 continue
             if in_var.dtype == src_dtype:
                 cast_name = in_var.name + '.cast_' + _dtype_to_str(dest_dtype)
@@ -84,7 +84,7 @@ def _insert_cast_op(block, op, idx, src_dtype, dest_dtype):
                         name=cast_name,
                         dtype=dest_dtype,
                         persistable=False,
-                        stop_gradient=False)
+                        stop_gradient=in_var.stop_gradient)
 
                     block._insert_op(
                         idx,
@@ -100,7 +100,7 @@ def _insert_cast_op(block, op, idx, src_dtype, dest_dtype):
             else:
                 if op.has_attr('in_dtype'):
                     op._set_attr('in_dtype', dest_dtype)
-    if src_dtype == core.VarDesc.VarType.FP32:
+    if src_dtype == core.VarDesc.VarType.FP32 and dest_dtype == core.VarDesc.VarType.FP16:
         for out_name in op.output_names:
             if op.type == 'batch_norm' and out_name != 'Y':
                 continue