Improve examples using Ignite Trains server

[vfdev-5]

🚀 Feature

A follow-up of #1334 and idea is to setup output_uri to TrainsSaver if we are using Ignite Trains server to upload artifacts.

[sdesrozis]

Note that Trains fileserver can be configured in trains.config in sdk.developpment.default_output_uri that means no modification in code.

TrainsSaver(..., output_uri=<path>) will forward output_uri to related Trains.Task.init(...).

[vfdev-5]

Note that Trains fileserver can be configured in trains.config in sdk.developpment.default_output_uri that means no modification in code.

This is nice ! In this case we can only update docs for the examples.