Make ExperimentClient.observe support single value

Why:

Formatting results as dict(name, type, value) is cumbersome and the most
common use case is observing a single result which is the objective.
This use case should be simplified.

How:

Support `client.observe(trial, value, name)` and still support
`client.observe(trial, results)`.

src/orion/client/experiment.py

@@ -9,6 +9,7 @@
 
 import inspect
 import logging
+import numbers
 import typing
 from contextlib import contextmanager
 from typing import Callable
@@ -594,7 +595,12 @@ def suggest(self, pool_size=0):
         self._maintain_reservation(trial)
         return TrialCM(self, trial)
 
-    def observe(self, trial, results):
+    def observe(
+        self,
+        trial: Trial,
+        results: list[dict] | float,
+        name: str = "objective",
+    ) -> None:
         """Observe trial results
 
         Experiment must be in executable ('x') mode.
@@ -603,10 +609,13 @@ def observe(self, trial, results):
         ----------
         trial: `orion.core.worker.trial.Trial`
             Reserved trial to observe.
-        results: list
+        results: list or float
             Results to be set for the new trial. Results must have the format
             {name: <str>: type: <'objective', 'constraint' or 'gradient'>, value=<float>} otherwise
             a ValueError will be raised. If the results are invalid, the trial will not be released.
+            If `results` is a float, the result type will be 'objective'.
+        name: str
+            Name of the result if `results` is a float. Default: 'objective'.
 
         Returns
         -------
@@ -628,6 +637,9 @@ def observe(self, trial, results):
         """
         self._check_if_executable()
 
+        if isinstance(results, numbers.Number):
+            results = [dict(value=results, name=name, type="objective")]
+
         trial.results += [Trial.Result(**result) for result in results]
         raise_if_unreserved = True
         try:

tests/unittests/client/test_experiment_client.py

@@ -906,6 +906,40 @@ def test_observe_under_with(self):
 
             assert trial.status == "completed"  # Still completed after __exit__
 
+    def test_observe_with_float(self):
+        with create_experiment(config, base_trial) as (cfg, experiment, client):
+            trial = Trial(**cfg.trials[1])
+            client.reserve(trial)
+
+            client.observe(trial, 10.0)
+            assert trial.status == "completed"
+            assert trial.objective.name == "objective"
+            assert trial.objective.type == "objective"
+            assert not client._pacemakers
+
+    def test_observe_with_float_and_name(self):
+        with create_experiment(config, base_trial) as (cfg, experiment, client):
+            trial = Trial(**cfg.trials[1])
+            client.reserve(trial)
+
+            client.observe(trial, 10.0, name="custom_objective")
+            assert trial.status == "completed"
+            assert trial.objective.name == "custom_objective"
+            assert trial.objective.type == "objective"
+            assert not client._pacemakers
+
+    def test_observe_with_invalid_type(self):
+        with create_experiment(config, base_trial) as (cfg, experiment, client):
+            trial = Trial(**cfg.trials[1])
+            client.reserve(trial)
+
+            with pytest.raises(TypeError):
+                client.observe(trial, "invalid")
+            assert trial.status == "reserved"
+            assert trial.objective is None
+            assert client._pacemakers[trial.id].is_alive()
+            client._pacemakers.pop(trial.id).stop()
+
 
 def test_executor_receives_correct_worker_count():
     """Check that the client forwards the current number count to the executor"""