Fix lstrip bug revealed by B005 lint (#18177)

Fix lstrip bug revealed by B005 lint (#18177)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18177
ghimport-source-id: fbbf915b66762fc88bc5b541464e71ba27500958

Stack from [ghstack](https://github.com/ezyang/ghstack):
* #18184 Fix B903 lint: save memory for data classes with slots/namedtuple
* #18181 Fix B902 lint error: invalid first argument.
* #18178 Fix B006 lint errors: using mutable structure in default argument.
* **#18177 Fix lstrip bug revealed by B005 lint**

lstrip() doesn't strip a prefix; it strips all of the characters
in the passed in string.  B005 lint revealed this.  Replaced with
substring operation.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14530873

fbshipit-source-id: 13b3438fcc3cce13b5110730dc3d0b528a52930f