Qibo

What's Changed
* Remove TrotterHamiltonian (426) stavros11
* Set threads warnings (443) stavros11
* Add qibojit backend (427) stavros11
* Make qibojit default backend and update warnings (436) stavros11
* adding qibojit to CI (437) scarrazza
* disabling parallel evaluation on darwin (438) scarrazza
* documentation update (435) scarrazza
* Set number of threads for qibojit (434) stavros11
* Fixes for cupy backend (432) stavros11
* Set threshold for using Metropolis custom op (428) stavros11
* Hamiltonian expectation using terms (424) stavros11
* Tensorflow not installed warning (425) stavros11
* icarusq (422) scarrazza
* Small Grover change (421) igres26
* Fix `set_device` test for GPU (417) stavros11
* updating qibotf docs (416) scarrazza
* fixing CI (420) scarrazza
* Load custom operators from external library (398) stavros11
* adding version checks for backends (412) scarrazza
* documentation upgrade (411) scarrazza
* Docs improvements (399) scarrazza
* removing zpow and czpow gates (409) scarrazza
* Complex numbers in sympy Hamiltonians (410) stavros11
* Adjusts all tests to new format (406) stavros11
* Set test backend in conftest (407) stavros11
* Remove matmuleinsum backend (404) stavros11
* refactoring for qibotf (400) scarrazza
* fixing tests for numpy/qibotf (403) scarrazza
* Reuploading fix (401) AdrianPerezSalinas
* Falqon (378) AdrianPerezSalinas
* Make matrices backend agnostic (402) stavros11
* moving distributed circuit into core (395) scarrazza
* backends auto-discovery (394) scarrazza
* documentation instructions (393) scarrazza
* Grover model (364) igres26
* Examples grover (389) AdrianPerezSalinas

What's Changed
* icarusq (422) scarrazza
* Small Grover change (421) igres26
* Fix `set_device` test for GPU (417) stavros11
* updating qibotf docs (416) scarrazza
* fixing CI (420) scarrazza
* Load custom operators from external library (398) stavros11
* adding version checks for backends (412) scarrazza
* documentation upgrade (411) scarrazza
* Docs improvements (399) scarrazza
* removing zpow and czpow gates (409) scarrazza
* Complex numbers in sympy Hamiltonians (410) stavros11
* Adjusts all tests to new format (406) stavros11
* Set test backend in conftest (407) stavros11
* Remove matmuleinsum backend (404) stavros11
* refactoring for qibotf (400) scarrazza
* fixing tests for numpy/qibotf (403) scarrazza
* Reuploading fix (401) AdrianPerezSalinas
* Falqon (378) AdrianPerezSalinas
* Make matrices backend agnostic (402) stavros11
* moving distributed circuit into core (395) scarrazza
* backends auto-discovery (394) scarrazza
* documentation instructions (393) scarrazza
* Grover model (364) igres26
* Examples grover (389) AdrianPerezSalinas

What's Changed
* Test that code does not contain `print` statements (391) stavros11
* fixing hep import in sphinx (390) scarrazza
* removing undesired prints (387) scarrazza
* renaming cma function (383) scarrazza

What's Changed
* Bell inequalities variational - Example (371) igres26
* Fix variational models for numpy backends (379) stavros11
* Improving performance for circuits with many gates (363) stavros11
* Use single gates.py file for all backends (372) stavros11
* Simplify backend switch (370) stavros11
* Fix CPU custom operators for more than 32 qubits (377) stavros11
* Update main benchmark script (375) stavros11
* Fix variational classifier (368) DiegoGM91
* Proposal for new model code structure (365) stavros11
* Conditioning gates on measurement outcomes (352) stavros11
* Standarizing optimizers input and output (358) scarrazza
* Multiple collapse measurements with single circuit call (355) stavros11
* Update semiclassical Shor example to use measurements with collapse (356) stavros11
* Updating multiline circuit draw (354) scarrazza
* Fix `.on_qubits()` with `.controlled_by()` gates (349) stavros11
* Use random seed for frequency custom operator (350) stavros11
* Fix Metropolis for sparse distributions (347) stavros11

What's Changed
* Fix issue with tf slicing (345) stavros11
* Hamiltonian expectation for density matrices (344) stavros11
* Measurement fixes (325) stavros11
* Fix controlled by Y gates for custom backend (343) stavros11
* Metropolis algorithm for sampling shots (332) stavros11
* Speeding up Metropolis for frequency (338) scarrazza
* Move state reshape in gates (340) stavros11
* Add nshot switcher (341) scarrazza
* PartialTrace gate for density matrices (335) stavros11
* Fixing circuit.draw misalignment (334) scarrazza
* Fix gap callback for degenerate Hamiltonians (329) stavros11
* Fix einsum in density matrix traceout (328) stavros11
* Make DistributedState custom operator free (323) stavros11
* adding Windows support (316) scarrazza
* Fix old tests for Windows (319) stavros11
* Detach custom operators (317) scarrazza
* Fix custom operator imports in tests (318) stavros11
* upgrading to tf 2.4.1 (315) scarrazza
* State abstraction (314) stavros11
* MaxCut Hamiltonian (311) scarrazza
* Symbolic Hamiltonian fixes (312) scarrazza
* Tests that run without Tensorflow (310) stavros11
* Circuit viewer (308) scarrazza
* small fixes (307) scarrazza
* Backend abstraction (303) stavros11
* Parallel circuit evaluation (304) scarrazza
* custom initial_state (305) scarrazza
* custom initial_state GPU (306) scarrazza
* Documenting custom VQC optimizations (298) scarrazza
* Updating wheels with cuda 11 (301) scarrazza
* Upgrading tf to 2.4.0 (280) scarrazza
* Create base callbacks (296) stavros11
* Fix test failure on GPU for parallel lbfgs (297) scarrazza
* Trainable flag for parametrized gates (294) stavros11

Major New Features

- Collapse gate.
- Shor (2n+3) example.
- qPDF model and example.
- Density matrix simulation using custom operators.
- Post-measurement bit-flip errors.
- Thermal relaxation error channel.
- Parallel optimization using multiprocessing for L-BFGS-B on CPU.
- Improved performance when repeating circuit execution.
- Replacing TensorFlow ThreadPool with OpenMP for custom operators on CPU.
- General code upgrades including tests, wheels and documentation.

Fixes
- Cost function for autoencoder tutorial.
- Circuit depth calculation.
- Backend return name.
- Trotter Hamiltonian from_symbolic and make_compatible.
- Scipy missing requirement in pip package.