[cmake] bump LibreTT tag to pull in victor-anisimov/Librett#8 and victor-anisimov/Librett#9

Author: evaleev

INSTALL.md

@@ -64,7 +64,7 @@ Compiling BTAS requires the following prerequisites:
 
 Optional prerequisites:
 - [CUDA compiler and runtime](https://developer.nvidia.com/cuda-zone) -- for execution on CUDA-enabled accelerators. CUDA 11 or later is required. Support for CUDA also requires the following additional prerequisites, both of which will be built and installed automatically if missing:
-  - [LibreTT](github.com/victor-anisimov/LibreTT) -- free tensor transpose library for CUDA, HIP, and SYCL platforms that is based on the [original cuTT library](github.com/ap-hynninen/cutt) extended to provide thread-safety improvements (via github.com/ValeevGroup/cutt) and extended to non-CUDA platforms by [@victor-anisimov](github.com/victor-anisimov) (tag 35db79a1acaa723f468e7a88a8cd17fa67baf09a).
+  - [LibreTT](github.com/victor-anisimov/LibreTT) -- free tensor transpose library for CUDA, HIP, and SYCL platforms that is based on the [original cuTT library](github.com/ap-hynninen/cutt) extended to provide thread-safety improvements (via github.com/ValeevGroup/cutt) and extended to non-CUDA platforms by [@victor-anisimov](github.com/victor-anisimov) (tag 6e6d0af6af7ffffb182bbeeaa970e5a3ad97b699).
   - [Umpire](github.com/LLNL/Umpire) -- portable memory manager for heterogeneous platforms (tag f9640e0fa4245691cdd434e4f719ac5f7d455f82).
 - [Doxygen](http://www.doxygen.nl/) -- for building documentation (version 1.8.12 or later).
 - [ScaLAPACK](http://www.netlib.org/scalapack/) -- a distributed-memory linear algebra package. If detected, the following C++ components will also be sought and downloaded, if missing:

external/versions.cmake

@@ -27,8 +27,7 @@ set(TA_TRACKED_MADNESS_PREVIOUS_VERSION 0.10.1)
 set(TA_TRACKED_BTAS_TAG 3c91f086090390930bba62c6512c4e74a5520e76)
 set(TA_TRACKED_BTAS_PREVIOUS_TAG 5a45699b78d0540b490c8c769b61033bd4d4f49c)
 
-set(LIBRETT_URL https://github.com/ValeevGroup/Librett)
-set(TA_TRACKED_LIBRETT_TAG 35db79a1acaa723f468e7a88a8cd17fa67baf09a)
+set(TA_TRACKED_LIBRETT_TAG 6e6d0af6af7ffffb182bbeeaa970e5a3ad97b699)
 set(TA_TRACKED_LIBRETT_PREVIOUS_TAG 68abe31a9ec6fd2fd9ffbcd874daa80457f947da)
 
 set(TA_TRACKED_UMPIRE_TAG f9640e0fa4245691cdd434e4f719ac5f7d455f82)