Typing fix

[jordan-schneider]

Type comments are not prevalent in the codebase, but some type inference can be done automatically (e.g. using class definitions). This type comment is syntactically incorrect in that it causes mypy to throw an error. python 2 backward compatible comments either have to be on a line with a variable assignment, on a line with a function signature, immediately after a function signature, or on each line of a multiline function signature. The existing comment is neither. This small PR moves the comment to right after the function signature, and adds the type for the itr parameter too.

Longer term it would be really nice to have a stub file, if only the auto-generated one, so that mypy can check the types of the classes used in this package.
@astooke

[codecov-io]

Codecov Report

Merging #147 into master will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #147   +/-   ##
=======================================
  Coverage   22.56%   22.56%           
=======================================
  Files         128      128           
  Lines        7987     7987           
=======================================
  Hits         1802     1802           
  Misses       6185     6185           

Flag	Coverage Δ
#unittests	22.56% <0.00%> (ø)

Impacted Files	Coverage Δ
rlpyt/samplers/base.py	80.00% <0.00%> (ø)

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