fix dtype missmatch error (#53712) (#53764)

Pcard-70458
cherry-pick #53712

Author: zhangting2020

python/paddle/static/amp/fp16_utils.py

@@ -629,12 +629,15 @@ def cast_model_to_fp16(
 
     def need_process(op):
         need_process = True
-        if op.type in ["cast", "create_py_reader", "read"]:
+        if op.type in ["create_py_reader", "read"]:
             need_process = False
         else:
             for attr_name in ['out_dtype', 'dtype']:
-                if op.has_attr(attr_name) and is_float_dtype(
-                    op.attr(attr_name)
+                # output type of some operators such as fill_constant will be determined by the attribute value.
+                #
+                if not op.has_attr('in_dtype') and (
+                    op.has_attr(attr_name)
+                    and is_float_dtype(op.attr(attr_name))
                 ):
                     need_process = False
 
@@ -667,6 +670,24 @@ def need_process(op):
                     "---- Add into keep_fp16_ops because the op in white_list ----"
                 )
             else:
+                # if cast in orgin program, we only modifiy attr and output's dtype to avoid dtype mismatch errors.
+                if op.type == 'cast':
+                    in_var = block._find_var_recursive(op.input('X')[0])
+                    out_var = block._find_var_recursive(op.output('Out')[0])
+                    op._set_attr('in_dtype', in_var.dtype)
+                    out_var.desc.set_dtype(paddle.dtype(op.attr('out_dtype')))
+                    _logger.debug(
+                        "---- op type: {}, in var [name: {} dtype: {}], out var [name: {} dtype: {}], attr [in_dtype {} out_dtype {}] ----".format(
+                            op.type,
+                            op.input('X')[0],
+                            in_var.dtype,
+                            op.output('Out')[0],
+                            out_var.dtype,
+                            op.attr('in_dtype'),
+                            op.attr('out_dtype'),
+                        )
+                    )
+                    continue
                 # divide others ops into fp16/fp32 sets according to promoting principle.
                 dst_dtype = dest_type
                 if not use_promote: