doc: graph: add document for gated mlp fusion

doc/graph/complex_fusion/gated_mlp.md

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+Gated Multi-Layer Perceptron (Gated-MLP) {#dev_guide_graph_gated_mlp}
+=====================================================================
+
+## Overview
+
+Gated Multi-Layer Perceptron (Gated-MLP) is a variant of MLP which is widely
+used as the Feed Forward Network (FFN) in many Transformer-based Large Language
+Models (LLMs).
+
+Typically, the FFN in Transformer architecture [1] is defined as a two layer MLP
+with a ReLU activation in between which can be replaced with other activations.
+
+\f[
+
+    FFN(src,W,V) = ReLU(src \cdot W) \cdot V
+
+\f]
+
+Gated Linear Unit (GLU) is adopted to replace the first linear layer to
+improve the quality of Transformer-based models [2]:
+
+\f[
+
+    GLU(src,W_1,W_2) = (src \cdot W_1) \otimes Sigmoid(src \cdot W_2) \\
+
+    FFN(src,W_1,W_2,V) = GLU(src,W_1,W_2) \cdot V
+
+\f]
+
+Where the \f$ src \cdot W_1 \f$ is usually called "FC (fully-connected) up",
+\f$ src \cdot W_2 \f$ is called "FC gate", and the last linear is called
+"FC down".
+
+Swish activation is further adopted to replace Sigmoid in the GLU to form
+swiGLU.
+
+\f[
+
+    Swish(x) = x \otimes Sigmoid(x) \\
+
+    swiGLU(src,W_1,W_2) = (src \cdot W_1) \otimes Swish(src \cdot W_2) \\
+
+    FFN(src,W_1,W_2,V) = swiGLU(src,W_1,W_2) \cdot V
+
+\f]
+
+The Gated-MLP based on swiGLU is also adopted in LLMs like LLaMA [3], Qwen [4],
+etc.
+
+## Gated-MLP patterns
+
+oneDNN supports Gated-MLP and its optimization through Graph API [5] by defining
+the graph, getting partition from the graph, and optimizing the kernels
+underneath. In general, a Gated-MLP pattern is defined as a directional acyclic
+graph (DAG) using oneDNN Graph API.
+
+### Floating-point Gated-MLP
+
+oneDNN defines floating-point (f32, bf16, and f16) Gated-MLP as follows. The blue
+nodes are required when defining a Gated-MLP pattern while the brown nodes are
+optional.
+
+![Gated-MLP pattern](images/fp-gated-mlp.png)
+
+1. The first MatMul on the top left calculates "FC up": \f$ src \cdot W_1 \f$.
+   See [MatMul](@ref dev_guide_op_matmul) operation in Graph API.
+2. The second MatMul on the top right calculates "FC gate": \f$ src \cdot W_2 \f$.
+3. The Activation node is optional. If required, it can be constructed with the
+   activation operations in Graph API, for example, [ReLU](@ref dev_guide_op_relu),
+   [GELU](@ref dev_guide_op_gelu), [Sigmoid](@ref dev_guide_op_sigmoid), and so on.
+   For Swish activation, the node can be constructed with the [Sigmoid](@ref dev_guide_op_sigmoid)
+   and [Multiply](@ref dev_guide_op_multiply) as below. You can also refer the
+   [Gated-MLP example](https://github.com/oneapi-src/oneDNN/tree/main/examples/graph/gated_mlp.cpp)
+   for Swish definition.
+
+   ![Swish Activation](images/gated-mlp-swish.png)
+
+4. The last MatMul on the bottom performs the "FC down" operation between the
+   GLU output and \f$V\f$.
+
+## Data Types
+
+oneDNN supports the floating-point Gated-MLP pattern with data types f32, bf16,
+and f16. You can specify the data type via the input and output data type fields
+of logical tensors for each operation. oneDNN does not support mixing different
+floating data types in a floating-point Gated-MLP pattern.
+
+The definition of the data types and support status on different CPU and GPU
+platforms follow the general description in @ref dev_guide_data_types.
+
+## Implementation limitations
+
+1. oneDNN primitive-based Gated-MLP is implemented as the reference
+   implementation on both Intel Architecture Processors and Intel Graphics
+   Products. In this case, floating-point Gated-MLP patterns are usually
+   implemented with three f32, bf16, or f16 matmul (with binary or eltwise
+   post-ops) primitives.
+2. The Gated-MLP patterns functionally supports all input shapes meeting the
+   shape requirements of each operation in the graph. For example, the `MatMul`
+   operation requires shape consistency for `k` dimension. The `Multiply`
+   operation requires the input tensors to have the same shape or the shapes can
+   be properly broadcasted based on the operation attribute.
+
+## Examples
+
+oneDNN provides a [Gated-MLP
+example](https://github.com/oneapi-src/oneDNN/tree/main/examples/graph/gated_mlp.cpp)
+demonstrating how to construct a typical floating-point Gated-MLP pattern with
+oneDNN Graph API on CPU and GPU with different runtimes.
+
+For applications where the weights of FC up and FC gate are combined as a single
+tensor, oneDNN also provides an
+[example](https://github.com/oneapi-src/oneDNN/tree/main/examples/graph/gated_mlp_wei_combined.cpp)
+demonstrating how to create the weight tensors for the pattern with the offsets
+and strides from the combined weight tensor.
+
+## References
+
+1. Attention is all you need, https://arxiv.org/abs/1706.03762v7
+2. GLU Variants Improve Transformer, https://arxiv.org/abs/2002.05202
+3. LLaMA: Open and Efficient Foundation Language Models, https://arxiv.org/abs/2302.13971
+4. Qwen Technical Report, https://arxiv.org/abs/2309.16609
+5. oneDNN Graph API documentation, https://oneapi-src.github.io/oneDNN/graph_extension.html