For generators returned from predict, coerce results to a string

[squidarth]

Describe the bug

If right now, in your predict function, your return a generator that yields non-string values, these values will not get streamed to a client. In fact, the client will just receive an empty response.

def predict(payload: Dict):
    def inner():
        for i in range(5):
              yield i
     return inner()

To Reproduce
Steps to reproduce the behavior:

Expected behavior

Since pretty much any python object responds to str(), we should just coerce the user's results to a string. This will be much more predictable & clearer to the user when they've done something they haven't intended, as opposed to seeing an empty response.

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Desktop (please complete the following information):

• OS: [e.g. macOS]
• Environment versions [e.g. python versions, framework/package versions]
• Truss version [e.g. 22]

Additional context
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