Add preliminary sphinx documentation

docs/source/trainer_api.md

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+# Trainer API
+
+The {class}`TTS.trainer.Trainer` provides a lightweight, extensible, and feature-complete training run-time. We optimized it for 🐸 but
+can also be used for any DL training in different domains. It supports distributed multi-gpu, mixed-precision (apex or torch.amp) training.
+
+
+## Trainer
+```{eval-rst}
+.. autoclass:: TTS.trainer.Trainer
+    :members:
+```
+
+## TrainingArgs
+```{eval-rst}
+.. autoclass:: TTS.trainer.TrainingArgs
+    :members:
+```