(bugfix)(torchx/runner) properly remove TORCHX_ prefix from env var before setting scheduler_params

Summary:
kwargs to `create_scheduler()` scheduler factory method can be specified via the env var `TORCHX_{ARGUMENT_NAME}=arg_value`. 

For example: `TORCHX_TIER=prod` would call the factory method as `create_scheduler(session_name, tier="prod")`.

Python's `str.strip("torchx_")` doesn't remove the prefix `torchx_` but rather strips any characters `t`, `o`, `r`, `c`, `h`, `x`, `_` from the front and back of the string. 

Change to using `str.removeprefix("torchx_")` which does what we intended it to.

Reviewed By: ethanbwaite

Differential Revision: D79473894

Author: kiukchung

torchx/runner/api.py

@@ -129,9 +129,9 @@ def __init__(
     def _get_scheduler_params_from_env(self) -> Dict[str, str]:
         scheduler_params = {}
         for key, value in os.environ.items():
-            lower_case_key = key.lower()
-            if lower_case_key.startswith("torchx_"):
-                scheduler_params[lower_case_key.strip("torchx_")] = value
+            key = key.lower()
+            if key.startswith("torchx_"):
+                scheduler_params[key.removeprefix("torchx_")] = value
         return scheduler_params
 
     def __enter__(self) -> "Self":