[PJRT] [XLA:GPU] Bump down % of memory available to XLA to 0.8

We observe crashes from not enough memory available to NCCL otherwise.

0.85 does not seem to do the trick either.

PiperOrigin-RevId: 508387705

Author: cheshire

xla/pjrt/gpu/gpu_helpers.h

@@ -50,7 +50,7 @@ struct GpuAllocatorConfig {
 
   // Only used if kind == kBFC. The maximum fraction of available memory to
   // allocate.
-  double memory_fraction = 0.9;
+  double memory_fraction = 0.8;
 
   // Only used if kind == kBFC. If true, the allocator will immediately allocate
   // the maximum amount allowed by `memory_fraction`. This reduces