Add symbol api for randn and fix shape issue for randn ndarray and symbol api

[ChaiBapchya]

Description

1. Randn is not currently present in symbol API
   Fixes Absence of randn function in Symbol API #12755

2. Currently randn gives issue when passed NDArray to the randn function parameters loc and scale

>>> mx.nd.random.randn((2, 3), loc=a, scale=a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/bapac/workspace/incubator-mxnet/python/mxnet/ndarray/random.py", line 223, in randn
    assert isinstance(loc, (int, float))
AssertionError

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Code is well-documented:
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

Original works as well
loc and scale scalar

>>> mx.nd.random.randn(2, 3, loc=5, scale=1)

[[7.2122064 5.774004  6.0434403]
 [6.1839256 6.891711  3.7652586]]
<NDArray 2x3 @cpu(0)>

loc and scale as NDArray

>>> a=mx.nd.array([[1,2],[3,4]])
>>> a

[[1. 2.]
 [3. 4.]]
<NDArray 2x2 @cpu(0)>
>>> mx.nd.random.randn(2, 3, loc=a, scale=a)

[[[[-0.771029    0.5486156   1.5793836 ]
   [-0.85608196 -0.9768796   0.7919808 ]]

  [[ 2.4888437   1.9256786   1.0245001 ]
   [ 1.9547654   3.1492283   4.9322524 ]]]


 [[[ 5.0588713   4.064883    6.2195086 ]
   [ 3.3605237   0.08666945  0.6729102 ]]

  [[ 0.8471296   6.967091   -1.8937755 ]
   [-0.29237127 -0.16993093 -1.3115396 ]]]]
<NDArray 2x2x2x3 @cpu(0)>

[ChaiBapchya]

@mxnet-label-bot add [Bug, Operator, Doc]

[access2rohit]

Are you sure that its for Symbolic as well ? If thats not a restriction shouldn't we add unittest for symbolic mode too ?

[ChaiBapchya]

Addressed comment. Verified unit-tests work with

$ nosetests tests/python/unittest/test_random.py
......
----------------------------------------------------------------------
Ran 23 tests in 2158.096s

OK

For these build flags
[✖ 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]

[apeforest]

LGTM. Please add unit tests as @kshitij12345 suggested.

[access2rohit]

LGTM

[roywei]

@ChaiBapchya gentle ping to trigger CI so we can merge. thanks!

[ChaiBapchya]

@apeforest @kshitij12345 addressed! Pl review. Thank you!

[ChaiBapchya]

@kshitij12345 @apeforest @roywei ready for review.