[v1.x] Save/Load Gluon Blocks & HybridBlocks

[samskalicky]

Description

v1.x branch version of PoC for saving & loading Gluon Blocks & HybridBlocks from #19535. Version for master branch is #19564. 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 names of blocks' children 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 of blocks' children 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.

[mxnet-bot]

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[leezu]

Must not change load_json -> fromjson in the 1.x branch due to API compatibility

[samskalicky]

Must not change load_json -> fromjson in the 1.x branch due to API compatibility

backed out the change

[leezu]

Thanks @samskalicky