vulkan: Add fusion support for RMS_NORM+MUL

[jeffbolznv]

• Add a use_count to ggml_tensor, so we can detect if an output is used more than once.
• Change the ggml-vulkan rms_norm shader to optionally multiply by another tensor.
• Add detection logic and basic fusion logic in ggml-vulkan.
• Add some testing support for fusion. Rather than computing one node at a time, allow for computing the whole graph and just testing one node's results. Add rms_norm_mul tests and enable a llama test.

I picked rms_norm+mul to fuse because it's commonly generated in llm_graph_context::build_norm.

[jeffbolznv]

@slaren I'd like to get your thoughts on this. How do we know that the user doesn't want to have all the intermediate results available to them? Seems like they'd need to opt in or out of fusion somehow... GGML_TENSOR_FLAG_OUTPUT seems like almost what we would want, but it's slightly different and doesn't seem to be used much?

[slaren]

Checking for GGML_TENSOR_FLAG_OUTPUT is enough. This flag guarantees that the tensor will not be overwritten by another computation, so it is necessary to set this flag if the user wants the output of a tensor. In cases like imatrix, the graph is evaluated one operation at a time, so fusion should never be active.

[slaren]

Precisely because getting details like this one right is not completely obvious, it would be good to have a function in ggml similar to this one that checks if multiple operations can be fused, that all the backends can use to implement operator fusion.

[jeffbolznv]

Yeah, I can move some of the logic there, but I think eventually some of the implementation details may differ between backends. Hopefully there's a way to have some common logic and some backend-specific logic.

[jeffbolznv]

OK, I've extracted out some common logic into a helper function.

[jeffbolznv]

Forgot to include perf results. Helps pp, but no meaningful change to tg.

before:
Z:\github\jeffbolznv\llama.cpp\build\bin\RelWithDebInfo>llama-bench -m C:\models\Phi-3-mini-4k-instruct-q4.gguf -fa 1 -n 128 -p 512 --prio 1
ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = NVIDIA GeForce RTX 4070 (NVIDIA) | uma: 0 | fp16: 1 | warp size: 32 | shared memory: 49152 | int dot: 1 | matrix cores: NV_coopmat2
| model                          |       size |     params | backend    | ngl | fa |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -: | --------------: | -------------------: |
| phi3 3B Q4_K - Medium          |   2.23 GiB |     3.82 B | Vulkan     |  99 |  1 |           pp512 |      6213.91 ± 37.97 |
| phi3 3B Q4_K - Medium          |   2.23 GiB |     3.82 B | Vulkan     |  99 |  1 |           tg128 |        140.52 ± 0.86 |

Z:\github\jeffbolznv\llama.cpp\build\bin\RelWithDebInfo>llama-bench -m C:\models\meta-llama-3-8b-instruct.Q4_K_M.gguf -fa 1 -n 128 -p 512 --prio 1
ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = NVIDIA GeForce RTX 4070 (NVIDIA) | uma: 0 | fp16: 1 | warp size: 32 | shared memory: 49152 | int dot: 1 | matrix cores: NV_coopmat2
| model                          |       size |     params | backend    | ngl | fa |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -: | --------------: | -------------------: |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |           pp512 |      3600.60 ± 15.70 |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |           tg128 |         84.19 ± 0.21 |

after:
Z:\github\jeffbolznv\llama.cpp\build\bin\RelWithDebInfo>llama-bench -m C:\models\Phi-3-mini-4k-instruct-q4.gguf -fa 1 -n 128 -p 512 --prio 1
ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = NVIDIA GeForce RTX 4070 (NVIDIA) | uma: 0 | fp16: 1 | warp size: 32 | shared memory: 49152 | int dot: 1 | matrix cores: NV_coopmat2
| model                          |       size |     params | backend    | ngl | fa |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -: | --------------: | -------------------: |
| phi3 3B Q4_K - Medium          |   2.23 GiB |     3.82 B | Vulkan     |  99 |  1 |           pp512 |      6456.74 ± 56.08 |
| phi3 3B Q4_K - Medium          |   2.23 GiB |     3.82 B | Vulkan     |  99 |  1 |           tg128 |        140.58 ± 1.12 |

Z:\github\jeffbolznv\llama.cpp\build\bin\RelWithDebInfo>llama-bench -m C:\models\meta-llama-3-8b-instruct.Q4_K_M.gguf -fa 1 -n 128 -p 512 --prio 1
ggml_vulkan: Found 1 Vulkan devices:
ggml_vulkan: 0 = NVIDIA GeForce RTX 4070 (NVIDIA) | uma: 0 | fp16: 1 | warp size: 32 | shared memory: 49152 | int dot: 1 | matrix cores: NV_coopmat2
| model                          |       size |     params | backend    | ngl | fa |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -: | --------------: | -------------------: |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |           pp512 |      3644.10 ± 12.41 |
| llama 8B Q4_K - Medium         |   4.58 GiB |     8.03 B | Vulkan     |  99 |  1 |           tg128 |         84.04 ± 0.22 |