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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[xauthulei]

@kwonmha , you missed the \n, please have try. Thanks

[xauthulei]

Here miss the \n

[kwonmha]

Thx @xauthulei .
Added \n.

[xauthulei]

/lgtm

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