[UX][Serve] More comprehensive target_qps_per_replica check

[cblmemo]

Fixes #3359

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)

1. Autoscaling enabled

service:
  readiness_probe:
    path: /health
    initial_delay_seconds: 20
  replica_policy:
    min_replicas: 1
    max_replicas: 2
    target_qps_per_replica: 1

$ sky serve up -n http @temp/svc.yaml
Service from YAML spec: @temp/svc.yaml
Service Spec:
Readiness probe method:           GET /health
Readiness initial delay seconds:  20
Replica autoscaling policy:       Autoscaling from 1 to 2 replicas with Target 1 QPS/replica
Spot Policy:                      No spot policy

2. Error (target_qps_per_replica not set)

service:
  readiness_probe:
    path: /health
    initial_delay_seconds: 20
  replica_policy:
    min_replicas: 1
    max_replicas: 2

$ sky serve up -n http @temp/svc.yaml
Service from YAML spec: @temp/svc.yaml
ValueError: Detect different min_replicas and max_replicas while target_qps_per_replica is not set. To enable autoscaling, please set target_qps_per_replica.

3. Fixed replicas when min==max

service:
  readiness_probe:
    path: /health
    initial_delay_seconds: 20
  replica_policy:
    min_replicas: 1
    max_replicas: 1
    target_qps_per_replica: 1

$ sky serve up -n http @temp/svc.yaml
Service from YAML spec: @temp/svc.yaml
Service Spec:
Readiness probe method:           GET /health
Readiness initial delay seconds:  20
Replica autoscaling policy:       Fixed 1 replica
Spot Policy:                      No spot policy

4. Autoscaling disabled

service:
  readiness_probe:
    path: /health
    initial_delay_seconds: 20
  replica_policy:
    min_replicas: 1
    max_replicas: 1

$ sky serve up -n http @temp/svc.yaml
Service from YAML spec: @temp/svc.yaml
Service Spec:
Readiness probe method:           GET /health
Readiness initial delay seconds:  20
Replica autoscaling policy:       Fixed 1 replica
Spot Policy:                      No spot policy

• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

Thanks @cblmemo!