add flatten_chains and the relevant tests

[houpc]

The function merge_all_chains() has a bug, as shown in the test/computation_graph.jl/line 302-308.

The added function flatten_all_chains() should fix this bug and be used in optimize!() to replace merge_all_chains().

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f0cee8b) 64.41% compared to head (b78ca2a) 64.35%.
Report is 1 commits behind head on computgraph.

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Additional details and impacted files

@@               Coverage Diff               @@
##           computgraph     #150      +/-   ##
===============================================
- Coverage        64.41%   64.35%   -0.07%     
===============================================
  Files               62       62              
  Lines             4353     4303      -50     
===============================================
- Hits              2804     2769      -35     
+ Misses            1549     1534      -15     

Files	Coverage Δ
src/computational_graph/ComputationalGraph.jl	100.00% <ø> (ø)
src/computational_graph/optimize.jl	94.81% <100.00%> (+8.56%)	⬆️
src/computational_graph/transform.jl	96.85% <100.00%> (-0.43%)	⬇️

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