Update model file extension (#1422)

* Update model file ext to ```.pth```

* Update docs

* Rename more

* Find model files

docs/source/training_a_model.md

@@ -33,7 +33,7 @@
     If you like to run a multi-gpu training using DDP back-end,
 
     ```bash
-    $ CUDA_VISIBLE_DEVICES="0, 1, 2" python TTS/bin/distribute.py --script <path_to_your_script>/train_glowtts.py
+    $ CUDA_VISIBLE_DEVICES="0, 1, 2" python -m trainer.distribute --script <path_to_your_script>/train_glowtts.py
     ```
 
     The example above runs a multi-gpu training using GPUs `0, 1, 2`.
@@ -122,7 +122,7 @@
 
     ```bash
     $ tts --text "Text for TTS" \
-          --model_path path/to/checkpoint_x.pth.tar \
+          --model_path path/to/checkpoint_x.pth \
           --config_path path/to/config.json \
           --out_path folder/to/save/output.wav
     ```