Allow port-access in the pipeline

[Fixstars-momoko]

Currently, the output in the form of Halide::Func[] allows access to each buffer after port mapping from the output port of a pipeline.

TODO:

• Add the feature to access each buffer by index so that each buffer can be used as an input for the following BB.
• Update the Python bindings along with the above change.

[Fixstars-momoko]

C++ Status: https://github.com/fixstars/ion-kit/pull/164

[xinyuli1204]

Python Status:167

[Fixstars-momoko]

https://github.com/fixstars/ion-kit/pull/164 https://github.com/fixstars/ion-kit/pull/167 https://github.com/fixstars/ion-kit/pull/172