[tune] lazy trials (#10802)

* Lazily fill trial queue

* Update interface

* Update end to end reporter test

* Removed `next_trials()` method

* Lint

* Print total number of samples to be generated in progress reporter. Allow infinite samples.

* Nit check

python/ray/tune/web_server.py

@@ -211,7 +211,10 @@ def RunnerHandler(runner):
             resource["trials"] = []
             trial_generator = BasicVariantGenerator()
             trial_generator.add_configurations({name: spec})
-            for trial in trial_generator.next_trials():
+            while not trial_generator.is_finished():
+                trial = trial_generator.next_trial()
+                if not trial:
+                    break
                 runner.add_trial(trial)
                 resource["trials"].append(self._trial_info(trial))
             return resource