Okei. I have checked my git repo and the scripts I used to install several versions of Pytorch by February 2019 looked like (for python 2.7, 3.5 and 3.7):
$conda install -y pytorch=1.0.0 torchvision cudatoolkit=10.0 -c pytorch
$conda install -y pytorch=0.4.0 torchvision cudatoolkit=9.0 -c pytorch

and so on. So things might have changed from then. In fact, I initially used your syntax but by that time that did not work. I mean:
$conda install -y pytorch=1.0.0 torchvision cuda100 -c pytorch
$conda install -y pytorch=0.4.0 torchvision cuda90 -c pytorch

Moreover, it seems strange that by typing  cudatoolkit=10.0 with version 1.0.0 what I get installed is cuda9. Moreover, some versions cannot be installed with conda and only with pip:
-python 3.5 cuda9 version 0.4.0
-python2.7 cuda9 version 0.4.0.
As far as I checked