fix: handle variable discover of x[i] where i is a variable

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[github-actions]

Benchmark Results (Julia vlts)

Time benchmarks

	master	540689d...	master / 540689d...
ODEProblem	4.1 ± 0.11 ms	4.1 ± 0.12 ms	1 ± 0.04
init	0.0452 ± 0.017 ms	0.0454 ± 0.017 ms	0.997 ± 0.52
large_parameter_init/ODEProblem	15.7 ± 2.1 ms	15.7 ± 2.1 ms	0.998 ± 0.19
large_parameter_init/init	0.0738 ± 0.028 ms	0.0705 ± 0.022 ms	1.05 ± 0.51
mtkcompile	6.35 ± 0.28 ms	6.34 ± 0.27 ms	1 ± 0.061
sparse_analytical_jacobian/ODEProblem	17 ± 1.9 ms	16.6 ± 0.79 ms	1.03 ± 0.13
sparse_analytical_jacobian/f_iip	0.052 ± 0.001 μs	0.053 ± 0.001 μs	0.981 ± 0.026
sparse_analytical_jacobian/f_oop	0.296 ± 0.043 ms	0.29 ± 0.0057 ms	1.02 ± 0.15
time_to_load	4.49 ± 0.022 s	4.32 ± 0.02 s	1.04 ± 0.0071

Memory benchmarks

	master	540689d...	master / 540689d...
ODEProblem	0.0393 M allocs: 2 MB	0.0393 M allocs: 1.98 MB	1.01
init	0.417 k allocs: 0.0698 MB	0.417 k allocs: 0.0698 MB	1
large_parameter_init/ODEProblem	0.325 M allocs: 11.4 MB	0.325 M allocs: 11.4 MB	1
large_parameter_init/init	0.605 k allocs: 0.172 MB	0.605 k allocs: 0.172 MB	1
mtkcompile	0.0621 M allocs: 3.37 MB	0.0621 M allocs: 3.38 MB	0.995
sparse_analytical_jacobian/ODEProblem	0.194 M allocs: 8.03 MB	0.194 M allocs: 8.03 MB	1
sparse_analytical_jacobian/f_iip	0 allocs: 0 B	0 allocs: 0 B
sparse_analytical_jacobian/f_oop	0.634 k allocs: 19.6 kB	0.634 k allocs: 19.6 kB	1
time_to_load	0.153 k allocs: 14.5 kB	0.153 k allocs: 14.5 kB	1

[github-actions]

Benchmark Results (Julia v1)

Time benchmarks

	master	540689d...	master / 540689d...
ODEProblem	5.19 ± 0.13 ms	5.52 ± 0.43 ms	0.94 ± 0.076
init	0.058 ± 0.019 ms	0.0582 ± 0.018 ms	0.997 ± 0.45
large_parameter_init/ODEProblem	23.8 ± 5.1 ms	24.9 ± 4.3 ms	0.958 ± 0.26
large_parameter_init/init	0.0758 ± 0.037 ms	0.0714 ± 0.029 ms	1.06 ± 0.67
mtkcompile	8.09 ± 0.54 ms	8.69 ± 0.57 ms	0.932 ± 0.087
sparse_analytical_jacobian/ODEProblem	22.5 ± 3.3 ms	24.4 ± 2.8 ms	0.92 ± 0.17
sparse_analytical_jacobian/f_iip	0.09 ± 0.01 μs	0.09 ± 0 μs	1 ± 0.11
sparse_analytical_jacobian/f_oop	0.118 ± 0.012 ms	0.118 ± 0.013 ms	0.996 ± 0.15
time_to_load	5.9 ± 0.043 s	5.67 ± 0.06 s	1.04 ± 0.013

Memory benchmarks

	master	540689d...	master / 540689d...
ODEProblem	0.0396 M allocs: 1.77 MB	0.0396 M allocs: 1.77 MB	1
init	0.416 k allocs: 0.0518 MB	0.416 k allocs: 0.0518 MB	1
large_parameter_init/ODEProblem	0.353 M allocs: 12.8 MB	0.353 M allocs: 12.8 MB	1
large_parameter_init/init	0.799 k allocs: 0.152 MB	0.799 k allocs: 0.152 MB	1
mtkcompile	0.0601 M allocs: 2.68 MB	0.0601 M allocs: 2.68 MB	1
sparse_analytical_jacobian/ODEProblem	0.188 M allocs: 7 MB	0.188 M allocs: 7 MB	1
sparse_analytical_jacobian/f_iip	0 allocs: 0 B	0 allocs: 0 B
sparse_analytical_jacobian/f_oop	0.848 k allocs: 27 kB	0.848 k allocs: 27 kB	1
time_to_load	0.145 k allocs: 11 kB	0.145 k allocs: 11 kB	1