[BUG] compare.py benchmark fails

[jrp2014]

Running the compare benchmark from benchmarks/python/comparative leads to failure.

> python compare
:
0.5914489247792871	celu --size 32x16x1024 --cpu
0.6487723472573234	log_sigmoid --size 32x16x1024
-0.2201783376280044	log_sigmoid --size 32x16x1024 --cpu
Traceback (most recent call last):
  File "/Users/jrp/Documents/AI/mlx/mlx/benchmarks/python/comparative/compare.py", line 17, in run_or_raise
    return float(result.stdout)
           ^^^^^^^^^^^^^^^^^^^^
ValueError: could not convert string to float: b''

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/jrp/Documents/AI/mlx/mlx/benchmarks/python/comparative/compare.py", line 214, in <module>
    compare_filtered("step --size 32x16x1024")
  File "/Users/jrp/Documents/AI/mlx/mlx/benchmarks/python/comparative/compare.py", line 89, in <lambda>
    compare_filtered = lambda x: compare(x.split() + rest) if _filter(x) else None
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrp/Documents/AI/mlx/mlx/benchmarks/python/comparative/compare.py", line 24, in compare
    t_torch = run_or_raise(["python", BENCH_TORCH] + args)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jrp/Documents/AI/mlx/mlx/benchmarks/python/comparative/compare.py", line 19, in run_or_raise
    raise ValueError(f"stdout: {result.stdout}\nstderr: {result.stderr}")
ValueError: stdout: b''
stderr: b'Traceback (most recent call last):\n  File "/Users/jrp/Documents/AI/mlx/mlx/benchmarks/python/comparative/bench_torch.py", line 450, in <module>\n    raise ValueError("Unknown benchmark")\nValueError: Unknown benchmark\n'

Expected behavior
The benchmark should run to conclusion.

Desktop (please complete the following information):

• MacOS 14.3.1
• Head of mlx repo