refactor: unify TensorItem and TensorSlice models

[mehmetozsoy-synnada]

Description

Closes #120.

What is Changed

Include the changes introduced in this PR.

• TensorItem and TensorSlice is unified

Checklist:

• Tests that cover the code added.
• Corresponding changes documented.
• All tests passed.
• The code linted and styled (pre-commit run --all-files has passed).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.55%. Comparing base (7b53394) to head (a10bfbe).
Report is 1 commits behind head on main.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
+ Coverage   86.52%   86.55%   +0.03%     
==========================================
  Files          61       61              
  Lines       15617    15593      -24     
==========================================
- Hits        13513    13497      -16     
+ Misses       2104     2096       -8     

Files with missing lines	Coverage Δ
...ithril/backends/with_autograd/common_primitives.py	67.18% <ø> (-0.51%)	⬇️
mithril/backends/with_autograd/jax_backend/ops.py	85.31% <ø> (ø)
mithril/backends/with_autograd/mlx_backend/ops.py	31.91% <ø> (ø)
...ithril/backends/with_autograd/torch_backend/ops.py	79.54% <ø> (ø)
...hril/backends/with_manualgrad/common_primitives.py	66.92% <ø> (+0.25%)	⬆️
...hril/backends/with_manualgrad/numpy_backend/ops.py	85.52% <ø> (ø)
...backends/with_manualgrad/numpy_backend/ops_grad.py	87.44% <ø> (+0.66%)	⬆️
mithril/framework/constraints.py	94.99% <ø> (-0.05%)	⬇️
mithril/framework/logical/essential_primitives.py	99.57% <100.00%> (-0.02%)	⬇️
mithril/framework/logical/model.py	97.48% <ø> (ø)
... and 2 more

... and 1 file with indirect coverage changes