Add vLLM v0.18.1 source tree with KV transfer abort fix

third_party/vllm/ now tracked in git for direct patch management.
Based on vLLM v0.18.1 release with one patch applied:

  vllm/v1/core/sched/scheduler.py:
    Replace fatal assert with graceful skip when KV transfer callback
    arrives for an already-aborted request during PD disaggregated serving.

Future vLLM modifications should be made directly in third_party/vllm/
and committed normally. The patches/ directory is kept as documentation
of what changed from upstream.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

third_party/vllm/docs/models/extensions/instanttensor.md

@@ -0,0 +1,31 @@
+# Loading Model Weights with InstantTensor
+
+InstantTensor accelerates loading Safetensors weights on CUDA devices through distributed loading, pipelined prefetching, and direct I/O. InstantTensor also supports GDS (GPUDirect Storage) when available.
+For more details, see the [InstantTensor GitHub repository](https://github.com/scitix/InstantTensor).
+
+## Installation
+
+```bash
+pip install instanttensor
+```
+
+## Use InstantTensor in vLLM
+
+Add `--load-format instanttensor` as a command-line argument.
+
+For example:
+
+```bash
+vllm serve Qwen/Qwen2.5-0.5B --load-format instanttensor
+```
+
+## Benchmarks
+
+| Model | GPU | Backend | Load Time (s) | Throughput (GB/s) | Speedup |
+| --- | ---: | --- | ---: | ---: | --- |
+| Qwen3-30B-A3B | 1*H200 | Safetensors | 57.4 | 1.1 | 1x |
+| Qwen3-30B-A3B | 1*H200 | InstantTensor | 1.77 | 35 | <span style="color: green">**32.4x**</span> |
+| DeepSeek-R1 | 8*H200 | Safetensors | 160 | 4.3 | 1x |
+| DeepSeek-R1 | 8*H200 | InstantTensor | 15.3 | 45 | <span style="color: green">**10.5x**</span> |
+
+For the full benchmark results, see <https://github.com/scitix/InstantTensor/blob/main/docs/benchmark.md>.