Plan file support

[chhwang]

• Completely separate model definition from implementation
• Plan file details full implementation
• Models and plans are serialized into JSON
• Deeper integration with MSCCL++
• A few bug fixes

[codecov]

Codecov Report

Attention: Patch coverage is 89.53810% with 265 lines in your changes are missing coverage. Please review.

Project coverage is 90.50%. Comparing base (e476238) to head (2e45981).

Files	Patch %	Lines
ark/model/model_graph_impl.cpp	84.72%	44 Missing ⚠️
ark/api/executor.cpp	93.43%	29 Missing ⚠️
ark/ops/ops_matmul.cpp	81.29%	26 Missing ⚠️
ark/api/tensor.cpp	41.37%	17 Missing ⚠️
ark/codegen.cpp	93.98%	13 Missing ⚠️
ark/model/model_op_arg.cpp	71.11%	13 Missing ⚠️
ark/model/model_offset.cpp	50.00%	12 Missing ⚠️
ark/model/model_op.cpp	90.55%	12 Missing ⚠️
ark/ops/ops_communication.cpp	89.71%	11 Missing ⚠️
ark/api/dims.cpp	89.79%	10 Missing ⚠️
... and 18 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
+ Coverage   86.95%   90.50%   +3.54%     
==========================================
  Files          85      103      +18     
  Lines        5843     3958    -1885     
==========================================
- Hits         5081     3582    -1499     
+ Misses        762      376     -386     

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[Binyang2014]

Overall looks good to me. Just have several questions. Will finish the review soon

[Binyang2014]

LGTM. I don't review all files. Since most of op related code are similar, just reviewed some of them