Pin transformers to fix v0.9.1 doctest (#2048)

### What this PR does / why we need it?
Pin transformers to fix v0.9.1 doctest

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
doctest passed


- vLLM version: v0.10.0
- vLLM main:
c657369841

Signed-off-by: Yikun Jiang <yikunkero@gmail.com>

tests/e2e/doctests/002-pip-binary-installation-test.sh

@@ -49,6 +49,12 @@ function install_binary_test() {
     pip install vllm=="$(get_version pip_vllm_version)"
     pip install vllm-ascend=="$(get_version pip_vllm_ascend_version)"
 
+    # TODO(yikun): Remove this when 0.9.1rc2 is released
+    # https://github.com/vllm-project/vllm-ascend/issues/2046
+    if [ "$PIP_VLLM_ASCEND_VERSION" == "0.9.1rc1" ] || [ "$PIP_VLLM_ASCEND_VERSION" == "0.9.0rc2" ] ; then
+        pip install "transformers<4.53.0"
+    fi
+
     pip list | grep vllm
 
     # Verify the installation