testing/bench.py: _format_value lacks dict support

## Problem

In `tile_kernels/testing/bench.py`, the `_format_value()` function handles `torch.dtype`, `tuple`, and `None`, but not `dict`:

```python
def _format_value(value):
if isinstance(value, torch.dtype):
return dtype_to_str(value)
if isinstance(value, tuple):
return 'x'.join(str(v) for v in value)
if value is None:
return 'None'
return str(value) # dict falls through here → ugly {'key': 'val'}
```

When a `dict` is passed, it falls through to `str(value)`, producing raw output like `{'topk': 2, 'hidden': 256}` — inconsistent with the `,`-joined `k=v` style used everywhere else in benchmark output.

## Expected Fix

Add dict handling that formats as `k1=v1,k2=v2`, consistent with `make_param_key`:

```python
if isinstance(value, dict):
return ','.join(f'{k}={_format_value(v)}' for k, v in sorted(value.items()))
```