batch-scheduler: fix the number of cross-vm links computation

[csegarragonz]

This PR fixes a bug in the logic to calculate the number of cross-VM links.

Before, we calculated the number of cross-VM links as the product of the number of executors per VM.
This is only true if we have only 2 VMs.

Take for example the two following scheduling decision: [1, 1, 1, 1] and [2, 2].
By doing the product, the former has a number of cross-VMs of 1, and the latter of 4, which means that the planner would prefer the former over the latter.

In fact, the right algorithm to calculate the number of cross-VM links is to, for each host, add all non-local executors in different hosts, and times it for the number of multiple hosts. And divide by two at the end to remove duplicates.

The updated algorithm gives a number of cross-VM links of 6 and 4 respectively, making the planner prefer the latter over the former.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d7d8e97) 81.79% compared to head (2dba1af) 81.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
+ Coverage   81.79%   81.87%   +0.08%     
==========================================
  Files         106      106              
  Lines        7997     8002       +5     
==========================================
+ Hits         6541     6552      +11     
+ Misses       1456     1450       -6     

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