Move main_classes to a separate folder

docs/source/main_classes/trainer_api.md

@@ -0,0 +1,17 @@
+# 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:
+```