[BUG] Fix the logic error of lo

[YangKai0616]

Currently running pytest -rA gpt_oss/triton/attention.py will report the following error:

PASSED attention.py::test_eq[0-None-0.125-64-8-8-128-1-1]
PASSED attention.py::test_eq[0-None-0.125-64-8-8-128-1-2]
PASSED attention.py::test_eq[0-None-0.125-64-8-8-128-128-1]
PASSED attention.py::test_eq[0-None-0.125-64-8-8-128-128-2]
PASSED attention.py::test_eq[0-None-0.125-64-8-8-32-1-1]
PASSED attention.py::test_eq[0-None-0.125-64-8-8-32-1-2]
PASSED attention.py::test_eq[0-128-0.125-64-8-8-128-1-1]
PASSED attention.py::test_eq[0-128-0.125-64-8-8-128-1-2]
PASSED attention.py::test_eq[0-128-0.125-64-8-8-128-128-1]
PASSED attention.py::test_eq[0-128-0.125-64-8-8-128-128-2]
PASSED attention.py::test_eq[0-128-0.125-64-8-8-32-1-1]
PASSED attention.py::test_eq[0-128-0.125-64-8-8-32-1-2]
SKIPPED [8] attention.py:217: too many queries
FAILED attention.py::test_eq[5-None-0.125-64-8-8-128-1-1] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-None-0.125-64-8-8-128-1-2] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-None-0.125-64-8-8-128-128-1] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-None-0.125-64-8-8-128-128-2] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-None-0.125-64-8-8-32-1-1] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-None-0.125-64-8-8-32-1-2] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-128-0.125-64-8-8-128-1-1] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-128-0.125-64-8-8-128-1-2] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-128-0.125-64-8-8-128-128-1] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-128-0.125-64-8-8-128-128-2] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-128-0.125-64-8-8-32-1-1] - AssertionError: Tensor-likes are not close!
FAILED attention.py::test_eq[5-128-0.125-64-8-8-32-1-2] - AssertionError: Tensor-likes are not close!

In the _attn_fwd kernel, there is a logical error in the calculation of the loop boundary lo: when start_q > 0 (e.g. start_q = 5), lo is incorrectly restricted to start_q (i.e., 5). This causes key positions 0 to start_q-1 (i.e., 0-4) to be skipped. However, these positions are valid under the causal mask (since query position start_q = 5 can attend to historical keys 0-4). In the reference implementation, these positions are correctly included. Therefore, regardless of whether BANDWIDTH is enabled, the loop starting point lo should begin from 0 (the absolute start of the key sequence) rather than start_q.