Making Features as a singleton for improved caching

[access2rohit]

Description

Caches Features by making it singleton.
Used Approach mentioned here: https://medium.com/@mrfksiv/python-design-patterns-2-the-singleton-f995c3198c8

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Testing

ubuntu@ip-172-31-82-110 ~/incubator-mxnet (singleton_feature) $ MXNET_TEST_COUNT=1 nosetests --logging-level=DEBUG --verbose -s tests/python/unittest/test_runtime.py
1
/home/ubuntu/anaconda3/lib/python3.6/site-packages/h5py/init.py:36: FutureWarning: Conversion of the second argument of issubdtype from float to np.floating is deprecated. In future, it will be treated as np.float64 == np.dtype(float).type.
from ._conv import register_converters as _register_converters
test_runtime.test_features ... 2
[✖ CUDA, ✖ CUDNN, ✖ NCCL, ✖ CUDA_RTC, ✖ TENSORRT, ✔ CPU_SSE, ✔ CPU_SSE2, ✔ CPU_SSE3, ✔ CPU_SSE4_1, ✔ CPU_SSE4_2, ✖ CPU_SSE4A, ✔ CPU_AVX, ✖ CPU_AVX2, ✖ OPENMP, ✖ SSE, ✔ F16C, ✖ JEMALLOC, ✔ BLAS_OPEN, ✖ BLAS_ATLAS, ✖ BLAS_MKL, ✖ BLAS_APPLE, ✖ LAPACK, ✖ MKLDNN, ✖ OPENCV, ✖ CAFFE, ✖ PROFILER, ✖ DIST_KVSTORE, ✖ CXX14, ✔ INT64_TENSOR_SIZE, ✖ SIGNAL_HANDLER, ✔ DEBUG, ✖ TVM_OP]
ok
test_runtime.test_is_singleton ... 2
2
ok
test_runtime.test_is_enabled ... 2
ok
test_runtime.test_is_enabled_not_existing ... 2
ok

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Ran 4 tests in 0.001s

[access2rohit]

@ChaiBapchya @apeforest please review

[access2rohit]

@mxnet-label-bot add[pr-awaiting-review]

[access2rohit]

@larroy please review

[apeforest]

LGTM

[larroy]

nice!!

[larroy]

LGTM, simple and nice.

[access2rohit]

@mxnet-label-bot add [pr-awaiting-merge]

[ChaiBapchya]

LGTM!
good to see caching implemented!
Brownie points for those lint fixes!

[wkcn]

Great! LGTM. : )