Add README for torch._inductor.runtime (pytorch#141492)

Signed-off-by: Edward Z. Yang <[email protected]>

Pull Request resolved: pytorch#141492
Approved by: https://github.com/jansel
ghstack dependencies: pytorch#141491

torch/_inductor/runtime/README.md

@@ -0,0 +1,18 @@
+# `torch._inductor.runtime`
+
+This folder contains code needed at runtime by the output code of
+Inductor.  The output code of Inductor will import `torch` and
+`torch._inductor.runtime`, but should not import from other files in
+`torch._inductor.*`.  Note that this code includes code that is
+needed to actually perform Triton compilation, but is not needed
+in the actual, final runtime execution of kernels.
+
+Runtime includes Triton/C++ generated code, which are compiled (sometimes in
+parallel) when the output code of Inductor is imported.  It also includes
+the autotuning code and heuristics to decide block sizes of generated code.
+
+One of the original motivations for this directory split was so that the Triton
+compile subprocesses could access Triton and our compiler support code while
+mocking out most of `torch`, which can take seconds to import (sometimes more
+than a Triton compile itself).  An abandoned prototype of this can be found
+[here](https://github.com/pytorch/pytorch/pull/124682/files).