message: Cache the return value of MatmulDoFP32ComputationFP16Input() (which reads an environment variable).
  Previously, we read and parsed an environment variable every time a Matmul kernel was launched on the GPU device.
diff: @@ -220,10 +220,11 @@ bool ShouldUseGemv(uint64 n) {
  bool GetCublasAutotuneComputationType(const DataType& dtype,
  se::blas::ComputationType* compute_type) {
  using se::blas::ComputationType;
  - bool use_f32_for_f16_computation = MatmulDoFP32ComputationFP16Input();
  switch (dtype) {
  case DT_HALF:
  case DT_BFLOAT16:
  + static bool use_f32_for_f16_computation =
  + MatmulDoFP32ComputationFP16Input();
  if (use_f32_for_f16_computation) {
  *compute_type = ComputationType::kF32;
  } else {
  