Cannot fine-tune a Hifigan model
**Describe the bug**

I tried to fine tune the hifigan model according to `FastPitch_Finetuning.ipynb`, but I encountered the following error:

```
Traceback (most recent call last):
  File "examples/tts/hifigan_finetune.py", line 28, in main
    trainer.fit(model)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 740, in fit
    self._call_and_handle_interrupt(
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 685, in _call_and_handle_interrupt
    return trainer_fn(*args, **kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 777, in _fit_impl
    self._run(model, ckpt_path=ckpt_path)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 1199, in _run
    self._dispatch()
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 1279, in _dispatch
    self.training_type_plugin.start_training(self)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/plugins/training_type/training_type_plugin.py", line 202, in start_training
    self._results = trainer.run_stage()
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 1289, in run_stage
    return self._run_train()
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 1311, in _run_train
    self._run_sanity_check(self.lightning_module)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/trainer/trainer.py", line 1375, in _run_sanity_check
    self._evaluation_loop.run()
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/loops/base.py", line 145, in run
    self.advance(*args, **kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/loops/dataloader/evaluation_loop.py", line 110, in advance
    dl_outputs = self.epoch_loop.run(dataloader, dataloader_idx, dl_max_batches, self.num_dataloaders)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/loops/base.py", line 145, in run
    self.advance(*args, **kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/loops/epoch/evaluation_epoch_loop.py", line 122, in advance
    output = self._evaluation_step(batch, batch_idx, dataloader_idx)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/loops/epoch/evaluation_epoch_loop.py", line 217, in _evaluation_step
    output = self.trainer.accelerator.validation_step(step_kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/accelerators/accelerator.py", line 236, in validation_step
    return self.training_type_plugin.validation_step(*step_kwargs.values())
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/plugins/training_type/ddp.py", line 444, in validation_step
    return self.model(*args, **kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1102, in _call_impl
    return forward_call(*input, **kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/torch/nn/parallel/distributed.py", line 886, in forward
    output = self.module(*inputs[0], **kwargs[0])
  File "/root/miniconda3/lib/python3.8/site-packages/torch/nn/modules/module.py", line 1102, in _call_impl
    return forward_call(*input, **kwargs)
  File "/root/miniconda3/lib/python3.8/site-packages/pytorch_lightning/overrides/base.py", line 92, in forward
    output = self.module.validation_step(*inputs, **kwargs)
  File "/root/NeMo/nemo/collections/tts/models/hifigan.py", line 226, in validation_step
    audio, audio_len, audio_mel = batch
ValueError: not enough values to unpack (expected 3, got 2)
```

**Steps/Code to reproduce bug**
Generate the mel files according to the notebook and run `python examples/tts/hifigan_finetune.py --config_name=hifigan.yaml model.train_ds.dataloader_params.batch_size=32 model.max_steps=1000 ~model.sched model.optim.lr=0.0001 train_dataset=./hifigan_train_ft.json validation_datasets=./hifigan_val_ft.json exp_manager.exp_dir=hifigan_ft +init_from_nemo_model=tts_hifigan.nemo trainer.check_val_every_n_epoch=10 model/train_ds=train_ds_finetune`

**Expected behavior**

A clear and concise description of what you expected to happen.

**Environment overview (please complete the following information)**

If NVIDIA docker image is used you don't need to specify these.
Otherwise, please provide:
 - Environment location: Cloud: [AutoDL](https://www.autodl.com/)
 - Method of NeMo install: From source, `git clone` and `pip install .[tts]`


**Environment details**

If NVIDIA docker image is used you don't need to specify these.
Otherwise, please provide:
PyTorch  1.10.0, Python  3.8, Cuda  11.3

**Additional context**

Add any other context about the problem here.

GPU: RTX 3090 * 2
VRAM: 24GB
CPU: 30 Cores AMD EPYC 7543 32-Core Processor
RAM: 160GB