upgrade to flash-attn 2.8.0.post2 (#2828)

* upgrade to flash-attn 2.8.0.post2

* use cu126 with torch 2.6

* seems vllm 0.8.5.post1 not compatible with cuda12.6.3 and torch 2.6

* cu126 + torch 2.6 as the default

* use cu126 for multigpu w torch 2.6 too

* drop vllm for now from ci for now

docker/Dockerfile-base

@@ -37,7 +37,3 @@ RUN git lfs install --skip-repo && \
     pip3 install awscli && \
     # The base image ships with `pydantic==1.8.2` which is not working
     pip3 install -U --no-cache-dir pydantic==1.10.10
-
-RUN if [ "$PYTORCH_VERSION" = "2.7.1" ] ; then \
-        pip3 install flash-attn==2.7.4.post1; \
-    fi