Add FlexiBLAS build support per #64752 (#64815)

Summary:
To enable building torch+dependencies, set WITH_BLAS=flexi BLAS=FlexiBLAS

Fixes https://github.com/pytorch/pytorch/issues/64752

Pull Request resolved: https://github.com/pytorch/pytorch/pull/64815

Reviewed By: jbschlosser

Differential Revision: D31997745

Pulled By: albanD

fbshipit-source-id: db208d59002f5896608a03132616400f09d972aa
diff --git a/setup.py b/setup.py
index a5fd33d..32fefeb 100644
--- a/setup.py
+++ b/setup.py
@@ -121,7 +121,7 @@
 #     one in this file; needed to build with other frameworks that share ONNX.
 #
 #   BLAS
-#     BLAS to be used by Caffe2. Can be MKL, Eigen, ATLAS, or OpenBLAS. If set
+#     BLAS to be used by Caffe2. Can be MKL, Eigen, ATLAS, FlexiBLAS, or OpenBLAS. If set
 #     then the build will fail if the requested BLAS is not found, otherwise
 #     the BLAS will be chosen based on what is found on your system.
 #