[v1.x] Add more onnx operator export unit tests

[josephevans]

Description

Fix onnx export for size_array by reshaping output from scalar to 1D tensor.

Adds onnx export unit tests for the following operators:

• rnn_param_concat
• size_array
• sample_multinomial

[mxnet-bot]

Hey @josephevans , Thanks for submitting the PR
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[Zha0q1]

LGTM!