Fix epsilon composition

[awni]

Fix bug with epsilon composition.

Tests

Running tests...
Test project /Users/awni/repos/gtn/build
Start 1: autograd_test
1/8 Test #1: autograd_test .................... Passed 0.87 sec
Start 2: creations_test
2/8 Test #2: creations_test ................... Passed 0.37 sec
Start 3: criterion_test
3/8 Test #3: criterion_test ................... Passed 0.57 sec
Start 4: functions_test
4/8 Test #4: functions_test ................... Passed 0.86 sec
Start 5: parallel_test
5/8 Test #5: parallel_test .................... Passed 0.81 sec
Start 6: graph_test
6/8 Test #6: graph_test ....................... Passed 0.44 sec
Start 7: utils_test
7/8 Test #7: utils_test ....................... Passed 0.33 sec
Start 8: rand_test
8/8 Test #8: rand_test ........................ Passed 0.50 sec

100% tests passed, 0 tests failed out of 8

Total Test time (real) = 4.74 sec

Benchmarks

TLDR not a huge impact, negligible for CTC.

Before fix

Timing composeForward ... 142.12 msec
Timing composeBackward ... 0.79 msec
Timing composeForwardSorted ... 9.53 msec
Timing ctcLoss ... 124.37 msec
Timing ctcGrad ... 21.44 msec
Timing ngramCtcLoss ... 5.47 msec
Timing ngramCtcGrad ... 0.61 msec
Timing ctcBatched ... 210.01 msec

After fix

Timing composeForward ... 144.14 msec
Timing composeBackward ... 0.8 msec
Timing composeForwardSorted ... 10.19 msec
Timing ctcLoss ... 131.27 msec
Timing ctcGrad ... 21.73 msec
Timing ngramCtcLoss ... 6.19 msec
Timing ngramCtcGrad ... 0.61 msec
Timing ctcBatched ... 218.7 msec