Switch ZML to use .host_pinned memory on ROCm, addressing missing host_unpinned memory error.

zml/platform.zig

@@ -89,7 +89,9 @@ pub const Platform = struct {
         const memory_target: pjrt.Memory.Kind = switch (memory) {
             .host_unpinned => switch (platform.target) {
                 // Cuda doesn't have host_unpinned.
-                .cuda => .host_pinned,
+                // ROCm doesn't seem to have it either.
+                // TODO(gwenzek): investigate why it was not forced before.
+                .cuda, .rocm => .host_pinned,
                 else => .host_unpinned,
             },
             inline else => |t| t,