[Docs] Add Relax VM architecture documentation (#19389)

- Add a dedicated architecture document (`docs/arch/relax_vm.rst`) for
the Relax Virtual
Machine, covering the compilation pipeline (Relax IR → VMCodeGen →
VMExecutable), the
4-opcode instruction set and encoding format, the register-based
execution model (VMFrame,
RunLoop, VMClosure dispatch), built-in operations, serialization, and
the Python-level
interface (direct invocation, stateful API, profiling, instrumentation).
- Refactor the inline VM section in `docs/arch/index.rst` into a brief
summary with a
cross-reference to the new page, and add `relax_vm` to the toctree.

Author: tlopex

docs/arch/index.rst

@@ -243,23 +243,18 @@ Relax Virtual Machine
 
 Relax defines *what* to compute — it is a graph-level IR that describes the operators and dataflow
 of a model. The Relax Virtual Machine (VM) handles *how* to run it — it is the runtime component
-that executes the compiled result. During compilation, ``tvm.compile()`` invokes ``VMCodeGen`` to
-translate Relax functions into a compact bytecode representation. The resulting ``VMExecutable``
-bundles the bytecode together with a constant pool and per-function metadata, and can be serialized
-to disk for deployment.
-
-The VM uses a register-based interpreter with an intentionally minimal instruction set — only four
-opcodes: ``Call``, ``Ret``, ``Goto``, and ``If``. The VM itself performs no mathematical computation;
-it only orchestrates control flow (function calls, conditional branches, loops). The actual
-compute-intensive work — matrix multiplications, convolutions, and other operators — is carried out
-by TIR functions that have been compiled down to native GPU/CPU kernels, or by external libraries
-such as cuBLAS and cuDNN. The VM dispatches to them through the PackedFunc mechanism. Internally the
-VM recognizes three function kinds: *PackedFunc* for external C/C++ functions, *VMFunc* for
-bytecode-interpreted Relax functions, and *VMTIRFunc* for compiled TIR kernels.
-
-On the Python side, users interact with the VM through ``relax.VirtualMachine(executable, device)``,
-which provides both a direct invocation interface and a stateful set-input / invoke / get-output
-interface suitable for RPC-based remote execution.
+that executes the compiled result. The VM uses a register-based interpreter with only four opcodes
+(``Call``, ``Ret``, ``Goto``, ``If``) and performs no mathematical computation itself — it
+orchestrates control flow while dispatching actual work to compiled TIR kernels or external
+libraries.
+
+See :ref:`relax-vm-arch` for the full architecture documentation, including the compilation
+pipeline, instruction set details, execution model, and Python interface.
+
+.. toctree::
+   :maxdepth: 1
+
+   relax_vm
 
 Disco: Distributed Runtime
 ^^^^^^^^^^^^^^^^^^^^^^^^^^