Fix get_prompt_template() for parameterized_model_parser

Getting this error:
```
Exception: Cannot get prompt template string from prompt input: attachments=[Attachment(data='https://s3.amazonaws.com/lastmileai.aiconfig.public/uploads/2024_1_10_18_41_31/1742/bear_with_honey.png', mime_type='image/png', metadata=None), Attachment(data='https://s3.amazonaws.com/lastmileai.aiconfig.public/uploads/2024_1_10_18_41_31/7275/fox_in_forest.png', mime_type='image/png', metadata=None)] data=None
```

When Sarmad did this fix in #866, he missed a callsite in the `parameterized_model_parser`. This was missed because at the time the image_2_text model parser was of type `ModelParser` instead of `ParameterizedModelParser`, which had to be converted in here to fix a bug: #877


## Test Plan

Before
<img width="1295" alt="Screenshot 2024-01-11 at 02 52 42" src="https://github.com/lastmile-ai/aiconfig/assets/151060367/c0c99f1e-2996-41f3-8274-c13ecc43f07e">


After

https://github.com/lastmile-ai/aiconfig/assets/151060367/7a10932c-8580-4584-8942-4c287f23fc82

python/src/aiconfig/default_parsers/parameterized_model_parser.py

@@ -117,13 +117,13 @@ def resolve_prompt_template(
  """
  return resolve_prompt_string(prompt, params, ai_config, prompt_template)
 
- def get_prompt_template(self, prompt: Prompt, aiConfig: "AIConfigRuntime") -> str:
+ def get_prompt_template(self, prompt: Prompt, aiConfig: "AIConfigRuntime") -> str | None:
  """
  An overrideable method that returns a template for a prompt.
  """
  if isinstance(prompt.input, str):
  return prompt.input
- elif isinstance(prompt.input, PromptInput) and isinstance(prompt.input.data, str):
+ if isinstance(prompt.input, PromptInput) and isinstance(prompt.input.data, str):
  return prompt.input.data
- else:
-  raise Exception(f"Cannot get prompt template string from prompt input: {prompt.input}")
+ # Not all model inputs are parameterizable (e.g. image inputs)
+ return None