Dockerfile torch fix (#987)

* add torch to requirements.txt at build time to force version to stick

* fix xformers check

* better handling of xformers based on installed torch version

* fix for ci w/o torch

docker/Dockerfile

@@ -19,7 +19,6 @@ RUN git clone --depth=1 https://github.com/OpenAccess-AI-Collective/axolotl.git
 WORKDIR /workspace/axolotl
 
 # If AXOLOTL_EXTRAS is set, append it in brackets
-RUN sed -i "s/torch==.*/torch==$PYTORCH_VERSION/" requirements.txt
 RUN if [ "$AXOLOTL_EXTRAS" != "" ] ; then \
         pip install -e .[deepspeed,flash-attn,$AXOLOTL_EXTRAS]; \
     else \