[Tune] Replace lambda default arguments (#46596)

This PR fixes instances where lambda functions are used as default
arguments. Since default arguments are evaluated at function definition
time, mutable objects can have unintuitive behavior; additionally, they
prevent our documentation from rendering correctly. This PR is part of
#45129, but has been split up to minimize codeowner impact.

Signed-off-by: pdmurray <peynmurray@gmail.com>

python/ray/tune/tests/test_trial_scheduler.py

@@ -1372,9 +1372,10 @@ def assertProduces(fn, values):
                 seen.add(fn()["v"])
             self.assertEqual(seen, values)
 
-        def explore_fn(
-            config, mutations, resample_probability, custom_explore_fn=lambda x: x
-        ):
+        def explore_fn(config, mutations, resample_probability, custom_explore_fn=None):
+            if custom_explore_fn is None:
+                custom_explore_fn = lambda x: x  # noqa: E731
+
             new_config, _ = _explore(
                 config,
                 mutations,