TH-2695 Accept symbolic `reps` in `tensor.tile`

Scan inplace opt

changing `assert` to `AssertionError`

This `assert` statement does not provide any error message making debugging this case hard. Moreover, `assert` is eliminated when using optimized python code (https://docs.python.org/2.7/reference/simple_stmts.html#the-assert-statement), introducing a potential bug into the system. Explicitly checking and raising an AssertionError will prevent this from happening.

Scan replace Allocs with AllocEmpty

Mixed: crash fix, doc, doc memory/speed trade-off, fix tests

Crash gpu and opt speed up

do not call max on variable with no elements

Add optdb.DB __contains__ operator