[mlir][sparse] updated setter/getter comments

[aartbik]

Summary

For the getters, it is bad practice to keep the reference
around for too long, as explained in the new comment

[aartbik]

refined

[gussmith23]

Thanks for this clarification :)

mlir/include/mlir/Dialect/SparseTensor/Utils/Merger.h

public:
   bool isDim(unsigned t, unsigned i, Dim d) const {
     assert(t < numTensors && i < numLoops);
     return dims[t][i] == d;
   }
 
   /// Returns true if any set bit corresponds to queried dim.
   bool hasAnyDimOf(const llvm::BitVector &bits, Dim d) const;
 
-  /// Setter
+  /// Dimension setter
   void setDim(unsigned t, unsigned i, Dim d) { dims[t][i] = d; }
 
-  /// Getters.
+  /// Convenience getters to immediately access the stored nodes.
+  /// Typically it is inadvisible to keep the reference around, as in
+  /// "auto er = merger.exp(e))", since insertions into the merger
+  /// may cause data movement and invalidate the underlying memory address.
   TensorExp &exp(unsigned e) { return tensorExps[e]; }
   LatPoint &lat(unsigned l) { return latPoints[l]; }
   SmallVector<unsigned, 16> &set(unsigned s) { return latSets[s]; }
 
 #ifndef NDEBUG
   /// Print methods (for debugging).
   void dumpExp(unsigned e) const;
   void dumpLat(unsigned p) const;