[Dy2St]Remove usless cast operation to speed up FP16 training (#46851) (#46998)

python/paddle/fluid/dygraph/dygraph_to_static/partial_program.py

@@ -599,16 +599,6 @@ class PartialProgramLayer:
                         == paddle.float16):
                     in_vars[i] = var.astype('float16')
                     in_vars[i].name = name
-                if (self.forward_program.global_block().has_var(name)
-                        and self.forward_program.global_block().var(name).dtype
-                        == paddle.float16):
-                    in_vars[i] = var.astype('float16')
-                    in_vars[i].name = name
-                if (self.backward_program.global_block().has_var(name)
-                        and self.backward_program.global_block().var(name).dtype
-                        == paddle.float16):
-                    in_vars[i] = var.astype('float16')
-                    in_vars[i].name = name
 
     @property
     def program(self):