[TUTORIALS] Minor fix for tutorial 06

[lancerts]

Based on @pytest.mark.parametrize("Z, H, N_CTX, HEAD_DIM", [(1, 2, 1024, 64)]) BATCH, N_HEADS, HEAD_DIM = 4, 32, 64 the HEAD_DIM is 64 in both pytest and benchmark, which triggers assertion failure from tl.static_assert(BLOCK_N <= HEAD_DIM) since BLOCK_N can be 128 in the tunning. Therefore, change the tunning size for BN as [32, 64].

Both pytest and benchmark runs fine on GPU after the fix.