Fix oneDNN fallback for concat with scalar

[bgawrych]

Description

In gluon-nlp repository following bug can be observer:

raise get_last_ffi_error()
1525E mxnet.base.MXNetError: Traceback (most recent call last):
1526E File "../src/ndarray/ndarray.cc", line 606
1527E MXNetError: oneDNN doesn't support 0 dimensions

PR: dmlc/gluon-nlp#1581
CI Run: https://github.com/dmlc/gluon-nlp/runs/4397405377?check_suite_focus=true

[mxnet-bot]

Hey @bgawrych , Thanks for submitting the PR
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[bgawrych]

@mxnet-bot run ci [centos-cpu, unix-cpu, unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu, centos-cpu, unix-cpu]