Document the correct setting of temp dir for MIOpen

[marlon-tobaben]

We have a problem with permission clashes. MIOpen detects if the user's HOME is on a shared FS, in which case it will use /tmp for the kernel cache and performance DB. This can be a problem when /tmp is not per-user as on LUMI compute nodes, so multiple users write to the same MIOpen directories.

According to LUMI AIF container maintainers something like this would work:

MIOPEN_DIR=$(mktemp -d)
export MIOPEN_CUSTOM_CACHE_DIR=$MIOPEN_DIR/cache
export MIOPEN_USER_DB=$MIOPEN_DIR/config

We should mention is more explicitly in the guide.

[marlon-tobaben]

At the moment, we use the above and it seems to work, but probably should fix this in the container at some point.