Can not get logs from raw deployment by web-app

[LuBingtan]

/kind bug

What steps did you take and what happened:

1. create an inference service

apiVersion: "serving.kubeflow.org/v1beta1"
kind: "InferenceService"
metadata:
  name: "flower-sample"
  annotations: 
    serving.kubeflow.org/raw: "true"
spec:
  predictor:
    tensorflow:
      storageUri: "gs://kfserving-samples/models/tensorflow/flowers"

2. Then I tried to get logs of the isvc by web-app, but got an error

What did you expect to happen: Get logs from raw deployment by web-app

Anything else you would like to add:

Environment:

• Istio Version: None
• Knative Version: None
• KFServing Version: master
• Kubeflow version: None
• Kfdef:[k8s_istio/istio_dex/gcp_basic_auth/gcp_iap/aws/aws_cognito/ibm]
• Minikube version: v1.18.1
• Kubernetes version: (use kubectl version): v1.20.2
• OS (e.g. from /etc/os-release): Ubuntu 18.04