[Serve][Feature] Ability to use mathematical operations in variable assignment

[JGSweets]

Ideally we would not have to use linux commands to do mathematical operations such as multiply or divide.

Below uses one of the examples from the Skypilot documentation to illustrate the current vs desired state.

---

Current State

# service.yaml
.
.
.
run: |
  conda activate vllm
  PIPELINE_PARALLEL_SIZE=2
  TENSOR_PARALLEL_SIZE=$(($SKYPILOT_NUM_GPUS_PER_NODE / 2 ))
  python -m vllm.entrypoints.openai.api_server \
    --tensor-parallel-size $TENSOR_PARALLEL_SIZE \
    --pipeline-parallel-size $PIPELINE_PARALLEL_SIZE \
    --host 0.0.0.0 --port 8080 \
    --model mistralai/Mixtral-8x7B-Instruct-v0.1

---

Ideal State

# service.yaml
.
.
.
run: |
  conda activate vllm=
  python -m vllm.entrypoints.openai.api_server \
    --tensor-parallel-size ${SKYPILOT_NUM_GPUS_PER_NODE / 2} \
    --pipeline-parallel-size 2 \
    --host 0.0.0.0 --port 8080 \
    --model mistralai/Mixtral-8x7B-Instruct-v0.1

${SKYPILOT_NUM_GPUS_PER_NODE / 2} could be altered, but ideally it would be something inline like this for cleanliness.