update docs for custom request router and policy

[abrarsheikh]

Description

1. Update docs
2. catch exception and redirect users to docs

Related issues

#56855

Additional information

Hard to write tests for this situation.

Manually verified that this is the right exception to catch

try:
    cloudpickle.loads(b'\x80\x05\x95\xbc\x03\x00\x00\x00\x00\x00\x00\x8c\x1bray.cloudpickle.cloudpickle\x94\x8c\x0e_make_function\x94\x93\x94(h\x00\x8c\r_builtin_type\x94\x93\x94\x8c\x08CodeType\x94\x85\x94R\x94(K\x01K\x00K\x00K\x03K\x03KCCnt\x00\xa0\x01\xa1\x00}\x01|\x01j\x02}\x02t\x03t\x04\x83\x01\x01\x00d\x01|\x02\x04\x00\x03\x00k\x01r*d\x02k\x00r:n\x04\x01\x00n\x0cd\x03d\x04d\x05i\x01f\x02S\x00d\x06|\x02\x04\x00\x03\x00k\x01rNd\x07k\x00r^n\x04\x01\x00n\x0cd\x08d\x04d\ti\x01f\x02S\x00d\nd\x04d\x0bi\x01f\x02S\x00d\x00S\x00\x94(NK\tK\x11K\x02\x8c\x06reason\x94\x8c\x0eBusiness hours\x94K\x12K\x14K\x04\x8c\x18Evening batch processing\x94K\x01\x8c\x0eOff-peak hours\x94t\x94(\x8c\x08datetime\x94\x8c\x03now\x94\x8c\x04hour\x94\x8c\x05print\x94\x8c\x04avro\x94t\x94\x8c\x03ctx\x94\x8c\x0ccurrent_time\x94\x8c\x0ccurrent_hour\x94\x87\x94\x8c\x1b/home/ubuntu/apps/policy.py\x94\x8c!scheduled_batch_processing_policy\x94K\x0eC\x10\x00\x03\x08\x01\x06\x01\x08\x02\x18\x01\x0c\x02\x18\x01\x0c\x03\x94))t\x94R\x94}\x94(\x8c\x0b__package__\x94N\x8c\x08__name__\x94\x8c\x08__main__\x94\x8c\x08__file__\x94h\x18uNNNt\x94R\x94h\x00\x8c\x12_function_setstate\x94\x93\x94h#}\x94}\x94(h\x1fh\x19\x8c\x0c__qualname__\x94h\x19\x8c\x0f__annotations__\x94}\x94(h\x14\x8c\x10ray.serve.config\x94\x8c\x12AutoscalingContext\x94\x93\x94\x8c\x06return\x94h\x04\x8c\x0cGenericAlias\x94\x85\x94R\x94\x8c\x08builtins\x94\x8c\x05tuple\x94\x93\x94h2\x8c\x03int\x94\x93\x94\x8c\t_operator\x94\x8c\x07getitem\x94\x93\x94\x8c\x06typing\x94\x8c\x04Dict\x94\x93\x94h2\x8c\x03str\x94\x93\x94h:\x8c\x03Any\x94\x93\x94\x86\x94\x86\x94R\x94\x86\x94\x86\x94R\x94u\x8c\x0e__kwdefaults__\x94N\x8c\x0c__defaults__\x94N\x8c\n__module__\x94h \x8c\x07__doc__\x94N\x8c\x0b__closure__\x94N\x8c\x17_cloudpickle_submodules\x94]\x94\x8c\x0b__globals__\x94}\x94(h\x0eh\x0e\x8c\x08datetime\x94\x93\x94h\x12h\x00\x8c\tsubimport\x94\x93\x94h\x12\x85\x94R\x94uu\x86\x94\x86R0.')
except (ModuleNotFoundError, ImportError) as e:
    print(f"caused by {e} {type(e)}")

❯ python policy.py
caused by No module named 'avro' <class 'ModuleNotFoundError'>

[angelinalg]

just a couple of style nits.

[eicherseiji]

Thanks @abrarsheikh!

For my understanding: because we also need to support Protobuf serialization, we can't do something like vLLM here and "serialize by value" to pass custom routers/policies, right?

[abrarsheikh]

"serialize by value"

this is exactly what we also doing https://github.com/ray-project/ray/blob/master/python/ray/serve/config.py#L237-L239

[eicherseiji]

this is exactly what we also doing https://github.com/ray-project/ray/blob/master/python/ray/serve/config.py#L237-L239

Oh duh. Okay makes sense, thanks for the reminder :)