hparams: sends 4 requests when hparams data is missing

[stephanwlee]

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:

   tensorboard/tensorboard/components/tf_backend/requestManager.ts

   Line 109 in 5214b08

   constructor(nSimultaneousRequests = 1000, maxRetries = 3) {

2. hparams dashboard uses the requqestManager:

   tensorboard/tensorboard/plugins/hparams/tf_hparams_backend/tf-hparams-backend.ts

   Lines 66 to 79 in 5214b08

   	if (this._useHttpGet) {
   	const encodedRequest = encodeURIComponent(JSON.stringify(request_proto));
   	const url =
   	this._apiUrl + '/' + methodName + '?request=' + encodedRequest;
   	return this._requestManager.request(url);
   	}
   	/* Use POST */
   	const requestOptions = new RequestOptions();
   	requestOptions.withCredentials = true;
   	requestOptions.methodType = HttpMethodType.POST;
   	requestOptions.contentType = 'text/plain';
   	requestOptions.body = JSON.stringify(request_proto);
   	const url = this._apiUrl + '/' + methodName;
   	return this._requestManager.requestWithOptions(url, requestOptions);

3. requestManager retries on non-200:

   tensorboard/tensorboard/components/tf_backend/requestManager.ts

   Line 244 in 5214b08

   if (req.status === 200) {

Ideal changes:

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