add alexnet

[fuqianya]

PR types

New features

PR changes

APIs

Describe

Add AlexNet to Paddle Vision (include network architecture, test case and doc).

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ fuqianya
❌ fuqian_client

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[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.