do not scale gradient in bf16 mode (#21428)

* no dot scale gradient in bf16 mode

* fix since args.fp16 might be none

* fixed typo

* typo

* only do if grad scaling is true

* self.amp_dtype == torch.float16 is true

* put back prop when fsdp is not none

src/transformers/trainer.py

@@ -595,27 +595,26 @@ class Trainer:
             if args.half_precision_backend == "cuda_amp":
                 self.use_cuda_amp = True
                 self.amp_dtype = torch.float16 if args.fp16 else torch.bfloat16
-                self.do_grad_scaling = True
+                #  bf16 does not need grad scaling
+                self.do_grad_scaling = self.amp_dtype == torch.float16
+                if self.do_grad_scaling:
                     if self.sharded_ddp is not None:
                         self.scaler = ShardedGradScaler()
                     elif self.fsdp is not None:
-                    if self.amp_dtype == torch.float16:
                         from torch.distributed.fsdp.sharded_grad_scaler import (
                             ShardedGradScaler as FSDPShardedGradScaler,
                         )
 
                         self.scaler = FSDPShardedGradScaler()
-                    else:
-                        self.do_grad_scaling = False
-                        self.use_cuda_amp = False
-                        self.amp_dtype = None
-
                     elif is_torch_tpu_available():
                         from torch_xla.amp import GradScaler
 
                         self.scaler = GradScaler()
                     else:
                         self.scaler = torch.cuda.amp.GradScaler()
+                elif self.fsdp is not None:
+                    self.use_cuda_amp = False
+                    self.amp_dtype = None
             elif args.half_precision_backend == "cpu_amp":
                 self.use_cpu_amp = True
                 self.amp_dtype = torch.bfloat16
@@ -669,7 +668,7 @@ class Trainer:
 
         # torch.compile
         if args.torch_compile and not is_torch_compile_available():
-            raise RuntimeError("Using torch.compile requires a nighly install of PyTorch.")
+            raise RuntimeError("Using torch.compile requires a nightly install of PyTorch.")
 
     def add_callback(self, callback):
         """