[Bugfix] Switch --rates CLI arg to handle a comma separated list of values (#433)

## Summary
Since the refactor, --rates will not allow a comma separated list of
values like --rates=1,2,3,4

## Details
This PR re-enables this feature, where a user can specify a static list
of rates to sweep through.

## Test Plan

- Testing locally

## Related Issues
- None

---

- [X] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [X] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Author: markurtz

src/guidellm/__main__.py

@@ -156,8 +156,9 @@ def benchmark():
 )
 @click.option(
     "--rate",
-    type=float,
-    multiple=True,
+    type=str,
+    callback=cli_tools.parse_list_floats,
+    multiple=False,
     default=BenchmarkGenerativeTextArgs.get_default("rate"),
     help=(
         "Benchmark rate(s) to test. Meaning depends on profile: "
@@ -383,7 +384,7 @@ def run(**kwargs):
         kwargs.get("data_args"), default=[], simplify_single=False
     )
     kwargs["rate"] = cli_tools.format_list_arg(
-        kwargs.get("rate"), default=None, simplify_single=True
+        kwargs.get("rate"), default=None, simplify_single=False
     )
 
     disable_console_outputs = kwargs.pop("disable_console_outputs", False)

src/guidellm/utils/cli.py

@@ -3,9 +3,34 @@
 
 import click
 
-__all__ = ["Union", "format_list_arg", "parse_json", "set_if_not_default"]
+__all__ = [
+    "Union",
+    "format_list_arg",
+    "parse_json",
+    "parse_list_floats",
+    "set_if_not_default",
+]
 
 
+def parse_list_floats(ctx, param, value):  # noqa: ARG001
+    """
+    Callback to parse a comma-separated string into a list of floats.
+    """
+    # This callback only runs if the --rate option is provided by the user.
+    # If it's not, 'value' will be None, and Click will use the 'default'.
+    if value is None:
+        return None  # Keep the default
+
+    try:
+        # Split by comma, strip any whitespace, and convert to float
+        return [float(item.strip()) for item in value.split(",")]
+    except ValueError as e:
+        # Raise a Click error if any part isn't a valid float
+        raise click.BadParameter(
+            f"Value '{value}' is not a valid comma-separated list "
+            f"of floats/ints. Error: {e}"
+        ) from e
+
 def parse_json(ctx, param, value):  # noqa: ARG001
     if value is None or value == [None]:
         return None