Better example code for models using chains framework

[nnarayen]

🚀 What

Small followup to #1346, but purely focused on better example code in error messages.

There are still some references to chains, but I think this is better overall.

Before:

$ truss push my_model.py --remote baseten-staging
ERROR ChainsUsageError: The user defined code does not comply with the required spec, please fix below:
/Users/nnarayen/Desktop/repos/baseten-local-deploy/my_model.py:26 (PassthroughModel.__init__) : If `Model` uses context for initialization, it must have `context` argument of type `<class 'truss_chains.definitions.DeploymentContext'>` as the last argument.
Got arguments: `[]`.
Example of correct `__init__` with context:
class HelloWorld(chains.ChainletBase):
    # chains.depends indicates that the HelloWorld chainlet depends on the RandInt Chainlet
    # this enables the HelloWorld chainlet to call the RandInt chainlet
    def __init__(self, rand_int=chains.depends(RandInt, retries=3)) -> None:
        self._rand_int = rand_int

    def run_remote(self, max_value: int) -> str:
        num_repetitions = self._rand_int.run_remote(max_value)
        return "Hello World! " * num_repetitions

After:

$ truss push my_model.py --remote baseten-staging
ERROR ChainsUsageError: The user defined code does not comply with the required spec, please fix below:
/Users/nnarayen/Desktop/repos/baseten-local-deploy/my_model.py:26 (PassthroughModel.__init__) : If `Model` uses context for initialization,
it must have `context` argument of type `<class 'truss_chains.definitions.DeploymentContext'>` as the last argument.
Got arguments: `[]`.
Example of correct `__init__` with context:
class HelloWorld(chains.ModelBase):
    def __init__(self, context: chains.DeploymentContext = chains.depends_context()):
        self._call_count = 0

    def predict(self, call_count_increment: int) -> int:
        self._call_count += call_count_increment
        return self._call_count

Next steps:

• Context marker still invokes _example_chainlet_code() directly, since we've mainly fixed the InitValidator callsites here.

💻 How

🔬 Testing

[marius-baseten]

Can you put both examples into a subdir e.g. truss_chains/reference_code/?

[marius-baseten]

Let's call it "reference" (to highlight it's for showcasing correct API) in distinction from truss-chains/examples which are actual use case examples.

[nnarayen]

Good idea!