fixed trainer.strategy=auto from None. (#7369)

Signed-off-by: Xuesong Yang <1646669+XuesongYang@users.noreply.github.com>

tutorials/tts/FastPitch_Finetuning.ipynb

@@ -294,7 +294,7 @@
                 "  model.train_ds.dataloader_params.batch_size=24 model.validation_ds.dataloader_params.batch_size=24 \\\n",
                 "  model.n_speakers=1 model.pitch_mean=152.3 model.pitch_std=64.0 \\\n",
                 "  model.pitch_fmin=30 model.pitch_fmax=512 model.optim.lr=2e-4 \\\n",
-                "  ~model.optim.sched model.optim.name=adam trainer.devices=1 trainer.strategy=null \\\n",
+                "  ~model.optim.sched model.optim.name=adam trainer.devices=1 trainer.strategy=auto \\\n",
                 "  +model.text_tokenizer.add_blank_at=true \\\n",
                 ")"
             ]
@@ -347,10 +347,10 @@
                 "  * We use a fixed learning rate of 2e-4.\n",
                 "  * We switch from the lamb optimizer to the adam optimizer.\n",
                 "\n",
-                "* `trainer.devices=1 trainer.strategy=null`\n",
+                "* `trainer.devices=1 trainer.strategy=auto`\n",
                 "  * For this notebook, we default to 1 gpu which means that we do not need ddp.\n",
                 "  * If you have the compute resources, feel free to scale this up to the number of free gpus you have available.\n",
-                "  * Please remove the `trainer.strategy=null` section if you intend on multi-gpu training."
+                "  * Please remove the `trainer.strategy=auto` section if you intend on multi-gpu training."
             ]
         },
         {