Add forward_tts docs

docs/source/models/forward_tts.md

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+# Forward TTS model(s)
+
+A general feed-forward TTS model implementation that can be configured to different architectures by setting different
+encoder and decoder networks. It can be trained with either pre-computed durations (from pre-trained Tacotron) or
+an alignment network that learns the text to audio alignment from the input data.
+
+Currently we provide the following pre-configured architectures:
+
+- **FastSpeech:**
+
+    It's a feed-forward model TTS model that uses Feed Forward Transformer (FFT) modules as the encoder and decoder.
+
+- **FastPitch:**
+
+    It uses the same FastSpeech architecture that us conditioned on fundemental frequency (f0) contours with the
+    promise of more expressive speech.
+
+- **SpeedySpeech:**
+
+    It uses Residual Convolution layers instead of Transformers that leads to a more compute friendly model.
+
+- **FastSpeech2 (TODO):**
+
+    Similar to FastPitch but it also uses a spectral energy values as an addition.
+
+## Important resources & papers
+- FastPitch: https://arxiv.org/abs/2006.06873
+- SpeedySpeech: https://arxiv.org/abs/2008.03802
+- FastSpeech: https://arxiv.org/pdf/1905.09263
+- FastSpeech2: https://arxiv.org/abs/2006.04558
+- Aligner Network: https://arxiv.org/abs/2108.10447
+- What is Pitch: https://www.britannica.com/topic/pitch-speech
+
+
+## ForwardTTSArgs
+```{eval-rst}
+.. autoclass:: TTS.tts.models.forward_tts.ForwardTTSArgs
+    :members:
+```
+
+## ForwardTTS Model
+```{eval-rst}
+.. autoclass:: TTS.tts.models.forward_tts.ForwardTTS
+    :members:
+```
+
+## FastPitchConfig
+```{eval-rst}
+.. autoclass:: TTS.tts.configs.fast_pitch_config.FastPitchConfig
+    :members:
+```
+
+## SpeedySpeechConfig
+```{eval-rst}
+.. autoclass:: TTS.tts.configs.speedy_speech_config.SpeedySpeechConfig
+    :members:
+```
+
+## FastSpeechConfig
+```{eval-rst}
+.. autoclass:: TTS.tts.configs.fast_speech_config.FastSpeechConfig
+    :members:
+```
+
+