Fix: Prevent crash from uninitialized last_id in convert_partition.py

[VbhvGupta]

Description

This PR fixes a critical bug that causes a crash in convert_partition.py when a partition contains zero nodes across all node types.

• Issue: Fixes [CRASH CONDITION] - Critical Bug - Uninitialized last_id Variable #7896

Problem

The last_id tensor was only assigned within an else branch of a loop that iterates over node types. If a partition had no nodes for any type, this else branch was never entered, leaving last_id uninitialized. This resulted in a crash when the uninitialized variable was subsequently passed to dist.all_gather.

Solution

The fix ensures that last_id is always initialized before being used:

1. A new integer variable, max_last_id, is initialized with the previous partition's last ID before the loop begins.
2. Inside the loop, max_last_id is updated to track the maximum node ID encountered in the current partition.
3. The last_id tensor is now created after the loop completes, using the final max_last_id.

This change guarantees that dist.all_gather always receives a valid tensor, preventing the crash and improving the robustness of the partitioning process, especially for small graphs.