fix a small type error on bf16+pp (#3441)

Co-authored-by: NOlatunji Ruwase <olruwase@microsoft.com>
Co-authored-by: NLogan Adams <114770087+loadams@users.noreply.github.com>

deepspeed/runtime/pipe/engine.py

@@ -243,7 +243,7 @@ class PipelineEngine(DeepSpeedEngine):
         self._force_grad_boundary = True
         if self.pipeline_enable_backward_allreduce:
             if self.bfloat16_enabled():
-                if self.zero_optimization_stage() < ZeroStageEnum().gradients:
+                if self.zero_optimization_stage() < ZeroStageEnum.gradients:
                     self._bf16_reduce_grads()
                 else:
                     raise NotImplementedError("PP+BF16 only work for ZeRO Stage 1")