Fix bug in SoftmaxOp, compatible with MXNet 2.0, add custom Op C++ helper, refactor omp context

[wkcn]

In MXNet 2.0, mx.sym.simple_bind has been renamed to mx.sym._simple_bind.

• [x] compatible with MXNet 2.0
• [x] add CustomOp C++ helper
• [x] Fix bug in Softmax Op
• [x] refactor omp context

[coveralls]

Pull Request Test Coverage Report for Build 828

• 2 of 3 (66.67%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+1.0%) to 86.857%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
mobula/glue/mx.py	1	2	50.0%
<!--	Total:	2	3	66.67%	-->

Files with Coverage Reduction	New Missed Lines	%
docs/tutorial/test_mul_op.py	2	84.15%
examples/MyFirstOP.py	2	76.79%
<!--	Total:	4		-->

Totals
Change from base Build 818:	1.0%
Covered Lines:	1738
Relevant Lines:	2001

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💛 - Coveralls