Save/Load Gluon Blocks & HybridBlocks

[samskalicky]

Description

master branch version of PoC for saving & loading Gluon Blocks & HybridBlocks from #19535. Version for v1.x is #19565. This PR is fundamentally different due to the changes on the master branch for block names/UUIDs compared to v1.x

This PR adds 2 new APIs to Gluon Blocks:

save

Saves the model parameters and a json structure of the model that stores the various names/UUIDs in order to later restore and match the saved parameters. This does not actually store the model. Users still need to recreate their model architecture using their Gluon code in Python.

It also stores the hybridized Symbol objects for HybridBlocks and associated settings so that it can be later reloaded.

load

Loads the model parameters and a json structure of the model. It restores the names/UUIDs of blocks in order to match the saved parameters.

Also restores hybridized Symbol objects in HybridBlocks so they do not need to be hybridized again. After calling the load API users can immediately run inference without needing to hybridize.

Other changes

• renames load_json function in Symbol namespace to fromjson to match the corresponding tojson API

[mxnet-bot]

Hey @samskalicky , Thanks for submitting the PR
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[leezu]

LGTM