Fix layer count in autoencoder detector

[summeraz]

Resolves #138
Resolves #146
Resolves #254

This PR fixes a problem with the number of layers in the AutoEncoder detector. In the current master, the input layer is duplicated due to a bug in the loop where the hidden layers are added. The issue is fixed here. I have also added a test to ensure that the number of layers matches expectations.

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[summeraz]

Hey @yzhao062, sorry for jumping in unannounced with this. I have been using the AutoEncoder detector in pyod recently (awesome library you've created!) and noticed a problem with the number of layers in the model. Browsing through the issues, it seems several others have noticed this as well. The fix was easy, so I figured I would open a PR and add a test.

[yzhao062]

sorry for being late on this~ I was in moving from my PhD to new faculty job and was totally losing track on this. Could you resubmit this PR to the development branch? I am not back to this!

[summeraz]

No problem. I've changed the base branch to the development branch. I don't see any automatic CI test runs, so I'll close and reopen to see if that triggers them. Edit: No 😢