btl/uct: add support for using an another memory domain to form connections #12822
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The UCT BTL looks for a connect-to-iface interface in each memory domain to form connections for connect-to-endpoint transports. For example, with ib the btl will pick the UD transport as the means to setup RC. While there are connection transports available (RDMACM) I chose using UD (etc) to support networks that did not necessarily provide a connection transport.
I am currently working with improving support for Open MPI on a RoCEv2 system that does not provide support for UD (yet). This breaks the assumption that there will always be a connect-to-ifact transport available in all memory domains. To fix this issue this change updates the detection logic to locate a suitable transport for making connections (tcp by default). If a memory domain does not have a suitable connection transport the alternate will be used instead. This has been tested on our broken-UD system and works well.
It a connection-only transport is not needed the extra transport module is destroyed and the in-memory domain connection transport is used.