PLAT-2174: Use gretelai/gpt-auto as pretrained_model for natural language gpt template (#447)

* Use `gretelai/gpt-auto`

* Return using TinyLlama

Author: anastasia-nesterenko

config_templates/gretel/synthetics/natural-language.yml

@@ -8,7 +8,7 @@ name: "natural-language-gpt"
 models:
   - gpt_x:
       data_source: "__temp__"
-      pretrained_model: "mistralai/Mistral-7B-Instruct-v0.2"
+      pretrained_model: "gretelai/gpt-auto"
       column_name: null
       params:
         batch_size: 4