[TTS] Global Style Tokens implementation in FastPitch doesn't follow the original paper
**Describe the bug**

The implementation of Global Style Tokens (GSTs) in FastPitch introduced in #6417 does not follow the prescription of the original [paper](https://arxiv.org/abs/1803.09017). In particular, the difference lies in the choice of the reference audio for a given training/validation sample `<text, ground_truth_audio>`:
- In the original paper, it is recommended to use the ground-truth audio as the reference audio
- In the NeMo implementation, the reference audio is chosen at random among audio samples from the same speaker

I discovered this after training a FastPitch model with GSTs and observing that the choice of the reference audio at inference time would have virtually no impact at all. More precisely, given a text T and **n** _different_ reference audios R_1, ..., R_n, passing <T, R_i> for any i in [1, n] would result in almost the exact same audio A. I say "almost" because the files appeared different when compared using `diff`, but they had the _exact same length_ and they sounded _exactly the same_ to the ear.

So what I did was to modify the [code](https://github.com/NVIDIA/NeMo/blob/v1.20.0/nemo/collections/tts/data/dataset.py#L704) that's responsible for selecting the reference audio in `nemo.collections.tts.data.dataset.TTSDataset.__getitem__`, effectively changing this:
`reference_audio = self.featurizer.process(self.data[reference_index]["audio_filepath"], ...)`
to this:
`reference_audio = self.featurizer.process(sample["audio_filepath"], ...)`

which is exactly what the GST paper prescribes. In this case, I observed that the choice of the reference audio at inference time _did_ make a difference, however small (although this is probably due to the fact that the training set I used was not very varied in terms of speaking styles).

My intuition is that if an audio A1 is used to compute the style embedding that is then used to condition the generation of the Mel spectrogram for another audio A2, the model will learn to ignore that style embedding because the information that can be extracted from A1 is useless to generate the spectrogram for A2. You could argue that this isn't quite true because A1 and A2 come from the same speaker, hence you could extract speaker information from A1 to generate the spectrogram for A2, but FastPitch already contains a SpeakerLookup module that takes care of encoding speaker information.

**Steps/Code to reproduce bug**

1. Train a FastPitch model using GSTs, for example using the following configuration file: `examples/tts/conf/fastpitch_align_44100_adapter.yaml`
2. Perform multiple inference steps where the input text is always the same, but the reference audio is different
3. Generate audios using a vocoder such as HiFi-GAN

**Expected behavior**

Choosing different reference audios at inference time results in different audios produced as output.

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

 - Environment location: Bare-metal
 - Method of NeMo install: pip install

**Environment details**

- OS version: Ubuntu 20.04.2
- PyTorch version: 2.0.1
- Python version: 3.8

**Additional context**

Add any other context about the problem here.
GPU model: RTX 3090 Ti
