Support for arm64 in mxnet-mnist image

[tenzen-y]

What this PR does / why we need it:
I modified Dockerfile for mxnet-mnist to support arm64. Also, I upgraded mxnet version to v1.9.1.
I have tested using the following commands:

$ # For linux/amd64
$ docker run -it --pull always --platform=linux/amd64 ytenzen/multiplatform-mxnet:latest
$ # For linux/arm64
$ docker run -it --pull always --platform=linux/arm64 ytenzen/multiplatform-mxnet:latest

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part-of #1900

Checklist:

• Docs included if any changes are user facing

[coveralls]

Coverage decreased (-0.09%) to 73.545% when pulling f05cbf0 on tenzen-y:support-arm64-for-mxnet-trial-image into ca903e2 on kubeflow:master.

[johnugeorge]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johnugeorge, tenzen-y

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