Fix exit train w/ memory test but without setting any limit

[wasertech]

By setting limit to 0 for train_impl, we can keep the memory test without encountering #2195 making #2199 obsolete.

See the logs for the test: https://gist.github.com/wasertech/7274eee6e6d9a599920949788b248bbf

We need to fix this parameter so it doesn't prevent the exit of the process.

[reuben]

limit=0 is equivalent to not doing any memory test at all. Instead, could you add a flag that disables the test so people have a workaround that doesn't require changing the code?

[wasertech]

@reuben checkout #2205. I'll close this PR and the others related for clarity.