Fix error in 'qml.data.load()` when using 'full' parameter value

[brownj85]

Context:
qml.data.load() did not properly handle specified parameters that come after 'full'. For example, the following
call should return all qspin datasets matching sysname='Heisenberg', periodicity='open', layout='2x2', for any value
of lattice:

qml.data.load('qspin',sysname='Heisenberg',periodicity='open',lattice='full',layout='2x2')

Instead, we get an exception, even though matching datasets do exist:

ValueError: layout value of '2x2' is not available. Available values are ['1x16', '1x4', '1x8']

This happens because the find() function fails as soon as it can't find a matching dataset for any of the parameters. When it checks the 'chain' lattice, it can't find '2x2' and throws an exception, even though '2x2' is available for 'rectangular'.

Description of the Change:
Foldermap.find() will only throw an exception if it cannot find any matching datasets for a given parameter. This does mean that the user can provide invalid parameters, as long as one of them matches. For example:

qml.data.load('qspin',sysname='Heisenberg',periodicity='open',lattice='full',layout=['2x2', '17x32'])

Will not result in an error.

Benefits:

• qml.data.load() can handle full parameters that come before specific parameters.

Possible Drawbacks:

• The user will not get immediate feedback for erroneous parameters
• Users are no logner guaranteed to get a dataset for every combination of (fully qualified) parameters. For example, qml.data.load('qchem', molname='H2', basis=['STO-3G', 'CC-PVDZ'], bondlength=['0.5', '0.7']) will return 4 datasets. But if one those is removed, 3 will be returned without throwing an error.

Related GitHub Issues:
Fixes #4270

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4c3ab72) 99.64% compared to head (b42047e) 99.64%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4663   +/-   ##
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  Coverage   99.64%   99.64%           
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  Files         376      376           
  Lines       33828    33833    +5     
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+ Hits        33707    33712    +5     
  Misses        121      121           

Files	Coverage Δ
pennylane/data/data_manager/foldermap.py	100.00% <100.00%> (ø)

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[timmysilv]

looks good! just clarifying something you said:

...basis=['STO-3G', 'CC-PVDZ'], bondlength=['0.5', '0.7']) will return 4 datasets. But if one those is removed, 3 will be returned without throwing an error.

if one what is removed? and which 3 will be returned? I'm not entirely sure what the expected behaviour is there.

[brownj85]

if one what is removed? and which 3 will be returned? I'm not entirely sure what the expected behaviour is there.

So before this PR, if H2_CC-PVDZ_0.7 was removed from the datasets bucket (and the foldermap), calling
qml.data.load('qchem', molname='H2', basis=['STO-3G', 'CC-PVDZ'], bondlength=['0.5', '0.7']) would cause an exception, because one of the requested parameter combinations doesn't exist. After this PR it'll return the other 3 datasets without an exception. An exception is only raised if none of the requested parameter combinations exist.

There's not really an 'expected' behaviour here (afaik) so we're not breaking any promises.

[timmysilv]

ahh gotcha - you mean removed from the actual database, makes sense! looks great

[DSGuala]

Looks good to me, not getting the error anymore 💯