Add LeNet example

[huningxin]

Fix webmachinelearning/webnn#70

To facilitate the review, a working version is hosted at: https://huningxin.github.io/webnn-samples/lenet/

This example is based on the foundation webnn-polyfill in webmachinelearning/webnn-polyfill#1

@anssiko @wchao1115 @pyu10055, please take a look. Thanks!

[anssiko]

Thanks looks great! Small nit on narrow screens, e.g. mobile: I suggest you use CSS media queries to position prediction results and controls underneath the image on narrow screens. Currently, they're overlaid on top of the image on mobile, and as such, unreadable.

[pyu10055]

THanks Ningxin, this looks good, is it possible to provide a work demo url to try this out?

[huningxin]

THanks Ningxin, this looks good, is it possible to provide a work demo url to try this out?

https://huningxin.github.io/webnn-samples/lenet/

[huningxin]

@anssiko , I fixed the UI issue on narrow screens. I also updated the live version with this fix. Please take a look.

A screenshot:

[anssiko]

Looks great now also on narrow screens, thanks!

[huningxin]

According to the resolution 01 of WebML CG call on 9/17

RESOLUTION: Land WebNN polyfill and samples PRs when existing review comments have been addressed, do not block on in-flight spec PRs and API design discussion

All of the review comments by @pyu10055 have been addressed, so I suppose it is ready to merge. @anssiko , WDYT?

[anssiko]

We’re ready to land. Thanks!

[huningxin]

Thanks @anssiko , I am going to merge this PR.