Make the Jenkins use different envs for each build / clean up after torch

[ezyang]

I'm not sure how it happened but here is a failed build https://build.pytorch.org/job/pytorch-jit-py2-linux/38/ which when rebuilt https://build.pytorch.org/job/pytorch-jit-py2-linux/39/ succeeded.

The proximal cause of error seems to have been that, somehow, the build job was picking up installed files from another torch installation. This is deeply weird, since I was under the impression that Jenkins only runs one job on a machine at a time. One easy way to prevent this from happening in the future is to have a separate Conda env.

There are other opportunities to practice better hygiene. Inspecting an environment we see:

torch                     0.2.0+f54da17             <pip>
torch                     0.2.0+97cf46e             <pip>
torch                     0.2.0+6e838bd             <pip>
torch                     0.2.0+5181717             <pip>
torch                     0.2.0+ec3a5aa             <pip>
torch                     0.2.0+ba8e3bd             <pip>
torch                     0.2.0+df0d4ae             <pip>
torch                     0.2.0+ccb5aab             <pip>
torch                     0.2.0+32b258a             <pip>
torch                     0.2.0+d663012             <pip>
torch                     0.2.0+2f372ec             <pip>
torch                     0.2.0+430e035             <pip>
torch                     0.2.0+bc57d5d             <pip>
torch                     0.2.0+897d500             <pip>
torch                     0.2.0+7e55978             <pip>
torch                     0.2.0+e023697             <pip>
torch                     0.2.0+2be5676             <pip>
torch                     0.2.0+3f13c32             <pip>
torch                     0.2.0+5219cfe             <pip>
torch                     0.2.0+0d467ab             <pip>
torch                     0.2.0+673abf9             <pip>
torch                     0.2.0+4fa50c1             <pip>
torch                     0.2.0+43b59b6             <pip>

these should get cleaned up.