hparams: sends 4 requests when hparams data is missing

Repro steps:

1. launch TB on an empty logdir
2. open your favorite dev tools
3. open the hparams dashboard
4. notice 4 requests to /data/plugin/hparams/experiment

Description: not a bug per say but this is an inefficiency.

Cause:

1. requestManager has a retry mechanism and # of retries are determined by the setting: https://github.com/tensorflow/tensorboard/blob/5214b0822af46de61091eca608c59ab2fd0fbdc2/tensorboard/components/tf_backend/requestManager.ts#L109
2. hparams dashboard uses the requqestManager: https://github.com/tensorflow/tensorboard/blob/5214b0822af46de61091eca608c59ab2fd0fbdc2/tensorboard/plugins/hparams/tf_hparams_backend/tf-hparams-backend.ts#L66-L79
3. requestManager retries on non-200: https://github.com/tensorflow/tensorboard/blob/5214b0822af46de61091eca608c59ab2fd0fbdc2/tensorboard/components/tf_backend/requestManager.ts#L244

Ideal changes:

• we retry only when error status is status < 100 || status >= 500

Start with tensorboard/components/tf_backend/requestManager.ts, especially the retry setting and non-200 handling, then inspect tensorboard/plugins/hparams/tf_hparams_backend/tf-hparams-backend.ts. Reproduce the empty-logdir case in the hparams dashboard and verify that retries occur only for statuses below 100 or at least 500, avoiding the four repeated requests.