!4592 fix optimizer tuple inpus issue

Merge pull request !4592 from wangqiuliang/resolve-optimizer-tuple-inputs-issue

mindspore/nn/cell.py

@@ -223,6 +223,15 @@ class Cell:
         else:
             object.__delattr__(self, name)
 
+    def cast_inputs(self, inputs, dst_type):
+        res = list()
+        for item in inputs:
+            if isinstance(item, tuple):
+                res.append(self.cast_inputs(item, dst_type))
+            else:
+                res.append(cast(item, dst_type))
+        return tuple(res)
+
     def __call__(self, *inputs, **kwargs):
         if context.get_context("mode") == context.GRAPH_MODE:
             if kwargs:
@@ -250,14 +259,10 @@ class Cell:
         cast_inputs = list()
         if hasattr(self, "_mindspore_flags"):
             if self._mindspore_flags.get('fp16'):
-                for item in inputs:
-                    cast_inputs.append(cast(item, mstype.float16))
+                cast_inputs = self.cast_inputs(inputs, mstype.float16)
             if self._mindspore_flags.get('fp32'):
-                for item in inputs:
-                    cast_inputs.append(cast(item, mstype.float32))
-        if cast_inputs:
-            cast_inputs = tuple(cast_inputs)
-        else:
+                cast_inputs = self.cast_inputs(inputs, mstype.float32)
+        if not cast_inputs:
             cast_inputs = inputs
         if self.enable_hook:
             output = self._hook_construct(*cast_inputs, **kwargs)