Add importing yaml library on mnist_e2e_on_prem.ipynb (#553)

[kwonmha]

What this PR does / why we need it: Added missing import yaml statement.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #553

Special notes for your reviewer:

Let me know if I should add release notes.

Release note:

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[kwonmha]

Thx @xauthulei . Added \n.

[xauthulei]

/lgtm

@jinchihe , it seems the Travis CI have some issue, would you check it please. Thanks