Remove host syncs from compute_dacs_segsum_triton_varlen

[jgcb00]

Problem

The chunk-mapping construction loops over packed sequences reading scalars
from GPU tensors element by element:

for i in range(num_sequences):
    start = int(cu_seqlens[i].item())          # GPU->CPU sync
    n = int(chunks_per_seq[i].item())          # GPU->CPU sync
    state_seq_mapping[chunk_start:chunk_end] = i                  # kernel
    state_chunk_in_seq[chunk_start:chunk_end] = torch.arange(...) # kernel

That is 2*num_sequences + 1 GPU→CPU synchronizations plus
2*num_sequences tiny slice-fill kernels per call. Each .item() stalls
the CPU on the GPU stream. Under packed-varlen inference serving this runs
once per layer per prefill batch: with 29 mamba3 layers and 64 packed prompts
that is ~3,700 pipeline stalls (~100–200 ms) per prefill wave, dominating
time-to-first-token. Varlen training forwards pay the same pattern once per
layer per micro-batch.

Fix — fully vectorized on device, zero syncs

Sequence i owns the contiguous global chunk slots
[cu[i]//C + i, cu[i]//C + i + len_i//C + 1). The starts are strictly
increasing (floor(a+b) >= floor(a) + floor(b)), so the owner of slot g is
searchsorted(range_starts, g, right=True) - 1, slots at/after the owner's
range end are inactive padding, and the local chunk index is
g - range_starts[owner]:

range_starts = cu_seqlens[:-1] // C + arange(NS)
range_ends   = range_starts + (cu_seqlens[1:] - cu_seqlens[:-1]) // C + 1
owner  = searchsorted(range_starts, arange(nchunks), right=True) - 1
active = arange(nchunks) < range_ends[owner]
state_seq_mapping  = where(active, owner, 0)
state_chunk_in_seq = where(active, g - range_starts[owner], sentinel)

The inactive-slot sentinel (ceil(len_0 / C), same as before) stays a
device-side scalar. ~7 small kernels replace the loop; no .item()/.tolist()
anywhere, so the call no longer serializes the CPU against the GPU stream —
which also matters under async scheduling, where any sync blocks pipelining
of the next step's host work.

The old per-sequence overflow assert is dropped: by the same floor
inequality the last range end is always <= nchunks, so it could never fire.

Validation / measurements

• Mapping tensors bit-identical to the original loop for: NS=1 short/long,
  lengths exactly multiple of C, single-token sequences, mixed lengths, and
  32 packed sequences.
• End-to-end greedy outputs of our mamba3-MIMO hybrid unchanged.
• In serving (vLLM, GH200): together with hoisting two smaller sync sites in
  our integration layer, prefill syncs dropped ~520 → ~7 per 8-prompt batch
  and TTFT at 256 concurrent requests dropped 1.31 s → 0.85 s.